So a declaration of war is made when you wanna go to war with someone else (duh).

For example, country A hates country B's guts. A declares war on B. A and B go to war. One side wins, unless the whole world gets embroiled in it, then things get way more complicated.

So why not skip the declaration?

Country A hates country B's guts. A carries out a sudden invasion. B is probably dead.

Is there a reason to declare war during a time period before modern history? (Outside of ethical reasons and things like a knight's honor)

It seemed to me that declaring war against another country you wish to invade is just a really stupid thing to do. It's like telling someone that you want to murder that you are gonna shoot them (thus giving them time to run or fight back).

Of course there might be a few problems, B:"Why are you amassing your army beside our border? You dare go to war with us?" A:"Nope, we definitely not going to war with you, we would have declared war if we wanted to." The element of surprise would definitely be lost if the enemy finds a large encampment of soldiers outside their border but if it was planned well, it could be a devastating invasion where B was unable to mobilize their soldiers well.

This is also assuming that there were no international laws that prohibits both the threat and the use of force in international conflicts, which have made declarations of war largely obsolete in international relations in the modern era.

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    $\begingroup$ I suggest you pay more attention to your Wiki link. In part it discusses exactly the issues you raise. $\endgroup$ Sep 21, 2016 at 14:01
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    $\begingroup$ You might want to go to history.se or law.se with this as either could have good information for you. $\endgroup$
    – Separatrix
    Sep 21, 2016 at 14:05
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    $\begingroup$ It's come to my attention that you have a large number of [units] near my borders. We mean no harm. Our [units] are merely passing through the area. $\endgroup$
    – Kevin L
    Sep 21, 2016 at 15:21
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    $\begingroup$ Also think that by the time the central authority of side B gets the declaration side A has already crossed the border and hit several locations. It makes Side A look/feel good while still maintaining the advantage of surprise; "Look, we sent our messenger across the boarder an hour before we invaded; we cant help it if your guards held him up at the gate." (eh, my timescale is crazy, but you get the idea) $\endgroup$
    – Marky
    Sep 22, 2016 at 4:07
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    $\begingroup$ @Marky Actually that's almost exactly what happened with Japan's attack on Pearl Harbor. The Japanese ambassador was instructed to deliver the declaration of war at 1pm, at which point the planes were already in the air. Due to problems in the Japanese embassy though, the declaration of war was not delivered until after 2pm, by which time the attack was well under way. $\endgroup$
    – Graham
    Sep 22, 2016 at 10:11

15 Answers 15

  • A declaration of war allows the recipient to surrender before interrupting both economies to form armies for invasion and defense.
  • A declaration of war establishes basis for closing borders, ceasing trade with the recipient, and expelling opposing citizens. It provides legal penalties for those who continue such trade or harbor such citizens. (I.e., the bureaucracy can handle violators rather than requiring the sovereign to hear and act.)
  • A declaration of war announces to third parties that a very dangerous situation is approaching and they may wish to leave either or both countries for the duration.
  • A (-n ideal) declaration of war will specify conditions under which hostility will cease. (See casus belli.) Again, perhaps the recipient will acquiesce without further hostilities. Such a declaration has the benefit of appearing to be a demand from an aggrieved party for redress rather than being an irrational attack by a lunatic. Neighbors who talk before attacking are much better neighbors than irrationally dangerous neighbors.
  • It announces an opportunity for third parties to join or oppose the declaration in force, aid, or word. In any case, you find out who your friends and who your not-so-friendlies are.
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    $\begingroup$ + invasion without declaration worsens the diplomacy with your allies or neutral countries. They will trust you less and fear more $\endgroup$
    – Zavael
    Sep 22, 2016 at 6:45
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    $\begingroup$ +1 for the last point. Real wars do not happen in a vaccuum containing just the 2 belligerents, but other nations as well.. and those do like the warning and courtesy of a formal declaration. Breaking the rules is a casus belli against A for them, just when A is busy invading B. $\endgroup$
    – Chieron
    Sep 22, 2016 at 9:32
  • $\begingroup$ I value my irrationally dangerous neighbors. Keeps out the riff-raff. $\endgroup$
    – jaxter
    Sep 25, 2016 at 21:49
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    $\begingroup$ @Zavael - Orderint dum metuant, anyone? $\endgroup$ Mar 19, 2017 at 14:04
  • $\begingroup$ @WhatRoughBeast :) yes, good point $\endgroup$
    – Zavael
    Mar 25, 2017 at 12:21

A declaration of war would be needed to raise troops. Before the modern era, there weren't standing armies in most countries. You had to pull labor off of crops/cattle and make them an army specifically for a war. And if a king didn't have his own resources directly, he'd have to levy troops from vassals. All that takes time, and you need your allies to know the enemy to help persuade them to fight.

Moreover, a surprise attack with an army is damned hard to do anyway -- armies move slow compared to messengers on horse. Surprise attacks are for units, not armies. It can be done, but (my opinion) the improbability of success probably makes the advantages of the declaration of war worth the loss of surprise.

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    $\begingroup$ Case in point - Pearl Harbor. Not a full scale invasion, but designed to cripple the US naval capabilities. The declaration didn't occur until after the fact, but the damage was done. $\endgroup$ Sep 21, 2016 at 16:24
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    $\begingroup$ Pearl Harbor was done with a standing military force. That's a modern era attack. My comments were entirely about feudal Europe environment. I agree -- if you have a standing force, the declaration does seem tactically poor. $\endgroup$
    – SRM
    Sep 21, 2016 at 23:52
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    $\begingroup$ Even with a standing army in modern times, a declaration would be good propaganda for their own country, assuming the leaders are in some way held responsible for their actions (via reelection, etc). Making it an honorable war instead of an invasion goes a long way with the public. As in "Saddam has WMDs" vs "we're gonna throw Saddam out because we don't like him being in power". $\endgroup$
    – Geobits
    Sep 22, 2016 at 1:51
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    $\begingroup$ That likely depends on how much the tactical advantage will actually matter. If the US invades Costa Rica, no amount of forewarning is going to help them fend off the attack, while a good propaganda piece may prevent them from gathering allies to their side (which could turn the tables a bit more). Definitely hard to measure, but I'm sure there's some nightmare of a bureaucratic matrix somewhere to solve it ;) $\endgroup$
    – Geobits
    Sep 22, 2016 at 3:53
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    $\begingroup$ Pearl Harbor : the declaration was tightly scheduled to happen just before the attack but the US kept the ambassador waiting for hours (forewarned by signals intelligence of what was about to happen), successfully engineering the event as a surprise attack (Bamford, "Puzzle Palace") to generate public outrage and support for the war. $\endgroup$ Sep 22, 2016 at 11:40

Right now you're paying attention only to A and B.

Declaring war is typically in large part about countries C through Z.

If countries C through Z see A's attack on B as reasonably justified, they're likely to sit back and let A and B fight things out on their own.

If, however, A is seen as excessively aggressive, and attacking B simply because it can get away with it, that poses a substantial threat to the security of all the other countries--if they let A take over B, A is likely to remain aggressive and after a few years of assimilating B's economy is likely to be considerably larger and more powerful as well.

Unless C through Z feel insulated from A's aggression (e.g., even A + B is still too puny to be a threat, or A and B are too isolated to threaten others) they're likely to consider A making a sneak attack on B a good reason to join the fight--and even if A is extremely powerful, the combination of everybody else is likely overwhelming.

Keep in mind as well that even if C through Z are relatively pacifistic, there's likely to be at least some "hawkish" element in each that would welcome an excuse to invade A and get access to its resources. As such, even if A's attack on B leads to little real concern, it's still entirely possible that others will use it as an excuse to invade A anyway.

Bottom line: "international laws of war" (or anything similar) are largely a codification of preexisting behavior. Unless A and B are the sole powers in existence, the basic forces that have led to passage of those laws will be at work, and push toward (at least roughly) similar behaviors.

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    $\begingroup$ and this is what Axis forces failed to do in WW2. They would have liked England to stay out. $\endgroup$
    – joojaa
    Sep 22, 2016 at 5:34
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    $\begingroup$ This is the best of the (few) correct answers. Declaring a war has next to nothing to do with the countries involved, but everything with the interaction of the declaring country and the rest of the world. $\endgroup$
    – Burki
    Sep 23, 2016 at 15:18

(Typing on my phone here, will elaborate more using a computer at a later time)

Yes you do get an advantage, but at the cost of being branded as a person using dubious means. Back then im guessing the norm was to declare war, thus doing a sneak attack will raise eyebrows of neighbouring countries.

This will not only lower their opinion of you potentially causing your country to get ostracized from trade and agreements, but it might even incite a joint attack by other countries against yours.

Another potential impact is the respect and faith of your country citizens, similar to the vietnam war incident America faced, choices made by a nation that is not in favour with its citizens can lead to internal conflict. This puts a real damper on the overall war effort.

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    $\begingroup$ In medieval Europe, more often it was used as a rallying cry to get all the other tribes/states to join your cause. It wasn't a stigma to be a warmonger. That's what successful states did regularly. Attack in summer, rest in winter, repeat for strong economy! $\endgroup$
    – SRM
    Sep 21, 2016 at 14:19
  • $\begingroup$ Interesting, imagine it was a more for honor and loot kind of time. I base my answer off leaning towards the modern era but right you are sir! $\endgroup$
    – Last
    Sep 21, 2016 at 14:35
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    $\begingroup$ Allies are a big one. You could have a bunch of "mutual defense" treaties which state something along the lines of "if you declare war on X, we will also declare war on X." An official declaration of war is then important to get your allies to contribute to the war effort. It's also easier to convince allies and un-aligned countries to join you when it's an "honorable" war, rather than an underhanded land-grab invasion. $\endgroup$
    – R.M.
    Sep 21, 2016 at 17:22
  • $\begingroup$ Yep i guess its all a matter of appearances, granted people are more agreeable with things that are socially acceptable/social norm. Though that being said if your overall intention id aligned well enough with theirs I would think you can still ally with them in a sneak attack. The chances of them accepting though might be hindered due to the reasons provided in the various excellent answers $\endgroup$
    – Last
    Sep 22, 2016 at 5:00
  • A properly declared state of war could trigger certain constitutional and legal provisions in country A. Reserve troops can be mobilized. The media can be censored. Troublemakers can be held without trial.
  • Troops from country A might have better chances of being treated as POWs if they are captured, especially if there was a low-level "dirty war" between A and B before and both sides got used to shooting spies.
  • As Drake mentioned, often the declaration of war had the form of an ultimatum. This could be done for propaganda only or genuinely in good faith. "Stop mobilizing your troops or we will go to war, too."
  • $\begingroup$ The constitutional trigger was the big one I was thinking of. I'm thinking of the U.S. and Don't know much about other nations, but if a nation follows a document like a constitution, there is likely a section in it about the powers of the government to user military power. To declare war, there might be a democratic process to determine if it will be done. Once approved, the declaration allows the government to do things it might not otherwise be allowed to do. (This has apparently failed now in the U.S. where we just send troops, planes and bombs without a declaration.) $\endgroup$
    – TecBrat
    Sep 22, 2016 at 15:54
  • $\begingroup$ Rome is a very good example of a country which had significant legal changes during a time of war. $\endgroup$
    – Jeutnarg
    Sep 26, 2016 at 3:58

Launching a surprise invasion is almost completely impossible. More so the closer you get to the modern era. People tend to notice when large armies gather, or pick up on communications signals. Villagers flee to get out of the path of the army, they warn others. The message just keeps going. Eventually it reaches someone that can do something to confirm the rumors and respond. Thus, surprise is lost. All this can happen very quickly, but you can not gather an army quickly, and even with modern equipment you can't move an army all that quickly. The logistics just make it impossible.

The 'declaration' isn't usually as simple as sending a message saying "We're at war". Usually it would likely boil down to nation A making demands of nation B, and being refused. After that, a military build up tends to make things pretty obvious.

Additionally, as mentioned above, if you suddenly lash out without a clear justification, your own people will be very unlikely to support it. Which (Even in a feudal society) has very serious consequences on your efforts.

  • $\begingroup$ Not sure about your second sentence. It seems a surprise attack is much more possible these days than previously. In modern times bombs, missiles, troop helicopters, etc. could be deployed with little to no warning. It seems to me that there is a legitimate advantage to not declaring war when using these technologies (though other answers give plenty of good reasons to do it). $\endgroup$
    – user16107
    Sep 23, 2016 at 10:20
  • $\begingroup$ You're referring to small units or individual strikes, not massing armies. These days it's even harder to mass an army and go unnoticed because of the shear volume of radio communications, in addition to Satellite intelligence. As an additional point, all of those things can easily be detected and intercepted. Missiles are very easily picked up by various radar systems and there are all kinds of ways to take them out. Bombs must be carried by Aircraft, and those are even easier to see than missiles. You couldn't get an entire army in helicopters, even then those show up well on radar. $\endgroup$
    – Lord Drake
    Sep 23, 2016 at 12:58

Honor was important in pre-modern society, certainly more so than now. Also, in societies with more absolute concentration of power, the ruler was synonymous with the country. Wars between countries could be more accurately seen as wars between kings in the pre-1500 world.

If you are going to prove yourself better than this other king, you have to show up and fight him honorably. To do that, you can't stab him in the back, so a declaration of war is needed.

Now the fights didn't really go honorably at all, what with all the pillaging and bribing nobles to switch sides, and murdering each other's peasants. But it was more important to maintain the fiction of honorable battle, and that was done by establishing a causus belli, at least in Europe. In other parts of the world, you see the same process, rulers would send notice that, "Hey, bro, I'm coming to take all your money and women, and then force you and everyone else in the world to acknowledge how powerful and awesome I am."

  • For bending the market, a declaration of war most importantly allows traders and merchants to act on the base that there's a war. Also enemies may start to spend more resources on military and army and that may be harmful on the long run.
  • For speculation, instead of deliberately damaging a foreign market your declaration of war is mainly to allow you change prices and have a direct income from that. In example when your country has a lot of weapons industries a call to the arms is beneficial for your economy regardless if you will fight or not.
  • For allowing civilians to retreat, if you have a economic target (in example a iron mine), people may just to decide to stay away from it
  • For selecting the place where to fight, if you interested is in some economic resource you force enemy army to defend a particular spot regardless if you will invade it for really, that allows to make tactical Attacks elsewhere.
  • For converting people to your religion, feared people may want to just join you, however it is more likely you obtain also the opposite effect, it definitely depends on your culture.
  • Because of political pressure, you do not intend really to invade or attack someone, but maybe you want to obtain a particular result, the defending country may counter-attack for that reason, because if they give what attacker wants, no one forbis the attacker will ask the same again
  • Symbolic act with consent of the other country, there's again no real intention of war but both countries have interests in appearing as "in war status" to other people and countries (in example they both have an advantage in a reciprocal embargo).
  • To turn off revolutions, feared people may be more willing to stay with current leaders if a war is preparing, this has been showed many times even in recent times. However if the war last too long people may suddenly become more willing to start a revolt. (In that sense, a flash war is good)

You cannot analyze a war if first you don't identify all parties that have an advantage from that. As you see there are reasons for which friends may be at war and enemies may be at peace. That what is weird about economics and most people just don't realize. After all rarely do nations leaders die, most times just civilians and soldiers die.

War & economics are bad. Especially in a totally free market


You need to seriously reconsider what war is about. The leaders of country A do not attack country B because as you say "Country A hates country B's guts."

Instead Country A attacks country B because either:

  • Victory presents personal gain for the leaders of country A.
  • War itself leads to personal gain for the leaders of country A
  • Not starting a war leads to personal loss for the leaders of country A.

War follows certain conventions, and breaking some of these conventions carries a risk of labeling you as a bad guy, which can reduce your prestige and it can also reduce popular support from the war, which threatens the leaders' power. For example, when the US recently attacked Iraq it's known that they did spend quite some effort to forge evidence of WMDs, to give them a just cause for starting the war. One reason they did so was so they wouldn't be labeled as warmongers, which would cost them prestige and popular support.

Aside from prestige there's the equally important aspect of allies, and popular opinion about the war in neutral countries. Consider that leaders of other nations may want to join the war on either side but can't do so because public opinion is against them - if they joined the war anyway, they would lose a lot of prestige, just like Tony Blair did. By transmitting the image that your war is a just war, you make it easier for other leaders to join your side of the war and harder for other leaders to join the enemy side of the war, without them losing public support in their countries - which will directly threaten their power.

In war, in every historical era, there are things that are considered bad form, such as:

  • Have sharpshooters focus on officers.
  • Kill prisoners.
  • Rape and pillage.
  • Use crossbows.
  • Eradicate a conquered city by slaughtering every living thing inside of it.
  • Attack hospitals.
  • Start wars without war declaration.
  • Do anything other than meet the enemy army in open field mass battles.

You'll notice that not a single one of the above is universal, and each depends on where and when the war took place. If you look at history a little bit more, you'll also notice that there are examples where each of these were broken, during times when it was considered bad form to do so. This was done because the perceived benefits outweighed the perceived cost. Sometimes that perception was right, other times it was not. Consider the last time the Crimea changed hands - there was no war declaration, there wasn't even a war, yet soldiers invaded, took the country, and countless conventions were broken.

Last but not least, as long as you pretend to follow the conventions, you can hope that the enemy also follows the conventions. When the convention was that peasants on the losing side are slaughtered and nobles are ransomed, as a noble it was a good thing for you if the enemy followed the latter part of that convention.


Your assumption here is that countries just start a war because "they hate each others' guts", but that is usually not the case. Sure, there might be emotions and personal relations involved, but in the end, you usually want to achieve a goal by waging war. Depending on the era and other circumstances, this can go from raiding for resources to conquering land to showing strength and improving your political position.

You usually do not want to go to war on your own, you want to achieve a goal and for that, you need to give your own people and your allies a justification, maybe even a motivation they can get behind. Also, even in a big war, you usually do not want to slaughter every last enemy citizen. In the long run, you want them to work for your economy, one way or the other. You do not want to give them too much reason to hate you and cause rebellions for decades.

In most democracies, it will be frowned upon when the government just declares war spontaneously, and other forms of government might not be happy about it either. Breaking the rules also tends to deteriorate your relations with other states, even your allies, who now have to be suspicious what crazy plan you come up with in the future.

Also, as others stated, it gives your enemy a chance to negotiate, especially when you appear stronger (which is usually the time you would chose to declare war) and have an opportunity to achieve your goals without actually fighting.

A "surprise invasion" might actually make sense for a small country with a very mobile military, but for bigger countries, it's usually impossible to organize something like that.

  • $\begingroup$ The first Gulf War was a surprise invasion- subterfuge ensured that and that was part of the largest military in world. $\endgroup$ Sep 21, 2016 at 22:31
  • $\begingroup$ "In most democracies, it will be frowned upon when the government just declares war spontaneously" OK but how does that square with the fact that the USA hasn't declared war on anybody since 1942 or something? $\endgroup$ Sep 22, 2016 at 10:03
  • $\begingroup$ @user2617804 : in what sense was the first Gulf War a surprise invasion? The invaders asked the protecting superpower's permission first. globalresearch.ca/… $\endgroup$ Sep 22, 2016 at 11:33
  • $\begingroup$ @DavidRicherby: I did not even know, but it looks like they had a lot of other wars that were at least "authorized by congress" and, as far as I can tell, supported by a large part of the population, at least when they started. $\endgroup$
    – Silverclaw
    Sep 22, 2016 at 12:06
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    $\begingroup$ @BrianDrummond While it was happening, people referred to the Iran-Iraq war as "the Gulf War" but, these days, people seem to have forgotten about that and refer to the liberation of Kuwait as "the Gulf War" (for example, that's what the Wikipedia page "Gulf War" is about). Confusingly, people also call it "the first Gulf War", even though it was the second one and no later war in the region is commonly called "the second Gulf War." $\endgroup$ Sep 22, 2016 at 12:13

Many countries, especially democracies, require both in law and social norms for a public debate to occur before going off to war. A ruling party or coalition government that hoodwinks the general population and sends its people to war without "selling" it to the general public first is not likely to survive re-election unless the war is a quick and resounding success.

In addition, if the executive branch of government has the supreme authority of the armed forces, as in America, then troops can be sent on the president's whim. However, the legislature often controls the budget, and if the executive decides to declare war unilaterally, the legislature can slash the budget for the armed forces. This almost never happens, because it looks bad politically to cut funding for the military during wartime, but if the war is largely seen as illegitimate and unpopular by the general population, then the legislators who cut short the executive's military incursions would have much to gain by standing up to an act of tyranny (which they will use to characterize the executive's actions, if they have any propaganda sense whatsoever).

This happens in autocracies/aristocracies as well, although it is much smaller in scope and less transparent. Even if the "supreme leader" has absolute authority, a dictator is only as strong as the people that keep them in power.

Often these people are significant players in the military and the economy, but the process is largely the same: people who support a given administration will want to know that their resources will be used effectively in an upcoming war, and will be most displeased if they are not consulted beforehand.


There are two things to this question:

  • First, why to declare war at all

  • Second, why to declare war before attacking

For instance, when Nazi Germany invaded the USSR in 1941, they declared war. But they did so about 4 hours after invasion started to provide for surprise attack.

Why declare war at all? Usually you want your population to know that you are in war, and you want neutral parties to keep from supporting and trade with your enemy (you can warn them you would attack their ships), and you want your allies to join you.

Why declare war before attacking? If you want either preserve your image internationally or you want to make an ultimatum. Ultimatum is the final demand which you present to your adversary that may avoid hostilities. Every war costs money and life and if you can get what you want without war, it is always better.

A third question is whether to declare war verbally or legally. This mainly has to do with domestic policies.


The advantage of declaring war is that it lets your far flung allies gather support and kickstart their own efforts.

These allies may have to convince their populations that this is a good idea (as implied in several answers here already), and may have to also make some rapid changes in their economies to cope with the demands of a war they might not have seen coming.

An additional advantage to declaring war (again in terms of alliances) is the morale boost it can give.

Have a look at the British Empire and Dominions at the start of the second world war (edit: and France - with it's colonial resources. I left it out. My bad): It wasn't just England declaring war on Germany, but Great Britain AND Australia AND Canada AND New Zealand AND the colonial administrations of India, Singapore, Kenya etc etc. Citizens of Britain in this case might not have felt alone in a fight with all the commonwealth and all of its resources lining up behind them.

  • $\begingroup$ How did you miss France? The only allied Country that actually did stuf till they got run over. $\endgroup$ Sep 23, 2016 at 7:03
  • $\begingroup$ Edited and amended! Thank you for the suggestion. $\endgroup$ Sep 24, 2016 at 21:56

A country will formally declare war if some sort of rule exists requiring such a declaration and the leadership making the declaration does not wish to challenge the legitimacy of those rules at the start of this war.

The rule can be formal or informal.

It can be a domestic rule or an international/transnational one.

It can be from a secular or religious authority.

It can be from a government or non-governmental organization.

It can be as simple as the dominant great power simply dictating terms to its clients and vassals ("If you're going to go to war, we expect you to follow these rules.")

The rule might exist for either offensive or defensive wars, or both.

There are lots of reasons why such a rule MIGHT exist (some of which are mentioned above,) but ultimately it comes down to the fact that some rule DOES exist, and the leaders don't want to challenge it.


"A semblance of legitimacy"

To many rulers with one eye on their place in history, a formal declaration rather than a sneak attack reflects better, and at the very least provides a 'semblance of legitimacy'. If things go wrong, you might be in a worse negotiating position, or rather, without one entirely, if it all began with a sneak attack.


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