So in reality, navies and air forces really are less effective than armies at projecting power and winning wars. The problem is that people live on land, and so seas and the air are only important as long as you can use them to access enemy territory and engage them on the ground. Air forces or navies operating independently have only succeeded in causing surrender in a single historical case, that of Japan in WW2, which required nuclear weapons and absolutely no other options for Japan to force them into surrender.

Transferring this to a science fiction context, space navies should generally not be very effective at causing planets to fall without an invasion unless they are completely willing to use weapons of mass destruction.

Invasions are likewise problematic in most cases, because the history of amphibious assaults shows that they are doomed to failure far more than they succeed for a variety of reasons. D-Day was a virtual fluke, only successful because of an unusual set of circumstances. Most successful amphibious assaults are like those of the Pacific War, against smaller secondary targets. It is also notable that the US didn't just land in Germany directly and landed in France after a deception and with bombing that slowed reinforcements until a large force could be built up.

Again transferring this idea to a science fiction context, invasions of major homeworlds or major city worlds should never really be possible. Only smaller colony worlds and outlying installations should easily fall unless several groups are able to gang up on another.

What could allow the state of affairs we see in science fiction, in which navies dominate and planetary invasions are generally possible?

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    $\begingroup$ "navies and air forces really are less effective than armies at projecting power and winning wars". Really? Why then the major power keep using carriers? $\endgroup$ – L.Dutch - Reinstate Monica Feb 11 at 7:57
  • $\begingroup$ This question seems very related, but coming from the other side: worldbuilding.stackexchange.com/questions/103768/… $\endgroup$ – Kyyshak Feb 11 at 8:28
  • $\begingroup$ navy is also useful for diplomacy $\endgroup$ – Kepotx Feb 11 at 9:18
  • $\begingroup$ The upside of coming in from space is you can drop anywhere on the planet, unlike navies which are restricted by distance. There is no such thing as a defensive line from space. $\endgroup$ – John Feb 11 at 11:15
  • $\begingroup$ @John We don't have a defensive line from space. That doesn't mean in the future there won't be one. Defensive satellites shooting deadly lasers to any non-authorized spaceship, for example? $\endgroup$ – Roberto Feb 11 at 13:12

The question fails on one major point: air and sea power are elements of Force Projection, allowing the user to bring forces to bear at the time and place of their choosing.

Nominally inferior powers like Athens vs Sparta in the Peloponnesian War, the Serenìsima Repùblica Vèneta vs the Ottoman Empire or Elizabethan England vs the Hapsburg Spanish Empire can challenge their far larger rivals through the use of force projection. Athens held against "Sparta and her allies" (backed by Persian money) for almost a decade after losing the flower of her army and fleet in the Sicilian campaign. The Serenìsima Repùblica Vèneta was highly competitive against the vastly larger Ottoman Empire, even when having to fight rival maritime powers like Genoa, and England clearly prevailed against the Spanish.

Force projection includes interdicting enemy trade, destroying resources at long range and landing troops or raiding parties whenever and wherever it is advantageous.

How this would work in a space environment really depends a lot on the basic assumptions you make. Planets are likely self sustaining enough to resist any blockade from orbit, but objects moving at interplanetary velocity will pack the energy of large nuclear weapons. If your space navy uses "jump points" or similar systems, then the analogues of naval choke points will exist (allowing for naval strategy), while "free" FTL (like Star Trek) would essentially give rise to "space vikings" since there is no practical way to secure the space around a planet from a ship dropping out of warp.

In more practical terms, controlling the ability to use planets as economic or resource bases of the larger polity through blockade is the most likely means of space warfare, gradually breaking the social, political and economic networks of the enemy polity necessary to continue waging war, even as individual planets are largely untouched. Once control over a planetary orbit is achieved, Space Marines may be necessary to make pin point raids to subdue or destroy critical facilities, but massive invasions involving billions of ground forces is unlikely in the extreme. Any polity capable of doing that can probably achieve any conceivable military, political or economic aim far more easily. These same calculations can be applied to moons, space colonies or space stations as well.

Space is not an ocean, and space warfare is not going to resemble the sorts of military actions we are familiar with. Once you have your basic background sketched out, the "how" military forces work will be much easier to answer.

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Personally i think you got the wrong idea about the effectiveness of Air and naval warfare.

Most of the war between Germany and Britain during world war 2 was fought in the skies. The effectiveness of destroying the opposing sides means of production, and strat bombing to reduce their morale is highly valuable under any military circumstances.

Translating this to space, Orbital bombardment would do miracles in a war and this is without using nukes. If you can hit them but they can't hit you their morale will plummet tot he point of surrender. In Star Wars the greatest fear was the Imperial navy not their ground forces. (Also the Death Star was a huge mistake as explained by Thrawn in the EU).

As for invasions, it would be possible due to the ability to cut of their resources. For example the Navy of the Union during the American Civil war is highly under appreciated seeing it effectively blocked confederate supplies coming in from across the sea. This severely weakened their economy and war effort. In space this will be harder due to a planets own natural resources but here comes the "joke" it is hard to defend your ENTIRE planet from a mobile force that can just move around the world at high speed bombing city after city making openings for invasion forces.

So in the long run a Space Navy will be the deciding factor in an interplanetary war...not ground forces.

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    $\begingroup$ Did you notice that the air raids failed completely in destroying morale? On the contrary, they make people hate their enemies even more. The only true benefit to bombing raids is to destroy strategically or tactically important targets, so that the ground forces can advance. $\endgroup$ – Burki Feb 11 at 11:50
  • $\begingroup$ Although the people stood strong, several high ranking officers started to support the idea of removing Hitler from power and suing for peace with the allies. This is because unlike civilians the officers see passed the propaganda and knew it was a war they could not win on the long term anymore. $\endgroup$ – A.bakker Feb 11 at 12:35
  • $\begingroup$ can you back your claim that this was due to the bombings, and not due to all the other things that hinted they would lose the war? $\endgroup$ – Burki Feb 11 at 14:14
  • $\begingroup$ As important as the US Navy was in blockading the Confederate coasts, Sherman still had to march his ground troops through the heart of the Confederacy. $\endgroup$ – RonJohn Feb 11 at 17:20

TLDR; navy and air force are often the deciding factors, and ground forces are only sent in to "mop up" after.

I think you don't have the right idea about the effectiveness of navy and air force. True, having solely navy or air force available for an invasion would not be very effective, and wars have not historically been won by bombing alone. But I would like to point out the Netherlands in the second world war. When they were invaded by the Germans by a ground assault, they flooded half their country to halt their progress, and the German ground assault could not continue. The only reasons the Dutch were still forced to surrender is because the Germans flew over the water and bombed our major cities. If this had been done without the accompanying ground assault, arguably this could still have been successful.

The same is true for the navy, they are capable of laying down extreme oppressive fire from great distances, and are much more mobile than tanks and ground assaults. Imagine having fortified your skies with flak cannons and your borders with bunkers, then suddenly the navy shows up out of the blue, obliterates your defences from afar with massive cannons, and sends in a ground force to mop up. This is much more effective than you would think.

Translating this to space navy: Modern drones have fairly high precision and rising. Nuking from orbit becomes much more feasible if you can precisely destroy one target while minimising unintended casualties. The only way to defend from that would be to build huge strong (underground) bunkers, and hole up in them. Which would definitely hurt your ability to effectively defend a planet, as you effectively imprison yourself.

When faced with a fleet of interstellar bombers, who systematically fly around your planet nuking every single defensive position you have, you could still try to resist. But you would be effectively crippled, and would be the equivalent of a tough guy waving a stick at the clouds. But without the stick because it got nuked.

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"The history of amphibious assaults shows that they are doomed to failure far more than they succeed for a variety of reasons. D-Day was a virtual fluke, only successful because of an unusual set of circumstances", says the question. This premise is false.

Oh really.

  • The cultural history of the European civilization begins with the Trojan War; for the sake of Helen, queen of Sparta, the Achaeans assembled a fleet and besieged Troy. ("Was this the face that launched a thousand ships / And burnt the topless towers of Ilium?", Cristopher Marlowe, Doctor Faustus, 1604.) The Achaeans won.

  • In 264 BCE, Appius Claudius Caudex invaded the Carthaginian-dominated Sicily across the strait of Messina. It was the first time the Romans attempted an sea-borne invasion. The Romans secured the port of Messana (modern Messina), which allowed them to bring reinforcements by sea. The Romans won, capturing all of Sicily except the kingdom of Syracuse, which had wisely remained neutral in the war.

  • In 204 BCE, Publius Cornelius Scipio, later called Africanus "The African", invaded the African territory of Carthage with 35,000 soldiers carried by "several hundred ships" (Wikipedia). The Romans won.

  • In 49 BCE, C. Julius Caesar invaded Greece (at that time held by Pompey) across the Adriatic. Caesar secured a foothold in Greece, was besieged by superior forces, but held fast; eventually Mark Anthony succeeded in bringing in reinforcement through the Pompeian blockade and forced the enemy to lift the siege. At the battle of Pharsalus Caesar won, opening the path towards his total domination of the Roman Republic.

  • In 43 CE, Aulus Plautius, on the orders of emperor Claudius, invaded Britain across the English Channel with Legio II Augusta (and most likely elements of two other legions). The invasion was successful, and the territory of what is now England remained a Roman province for the next four centuries.

  • In 711, Tariq ibn Ziyad, the Muslim governor of Tangier, invaded Visigothic Hispania across the strait which now bears his name -- Jabal Ṭāriq, the Mount of Tariq. The Muslims won, and Al-Andalus continued as a Muslim, Arabic-speaking country troughout most of the Middle Ages.

  • In 865, the Great Heathen Army invaded Britain across the North Sea. The Danes were victorious, successfully establishing dominion over north-eastern England, an area known as Danelaw.

  • In 1066, William the Bastard, duke of Normandy, invaded England across the Channel. He famously won at Hastings and became king of England.

  • At the beginning of the 13th century, the Venetians successfully invaded Crete, which at the time was (mostly) held by the Genoese. Crete remained part of the Venetian Stato da Màr for four centuries. (And in 1941 the Germans successfully invaded Crete, which at the time was held by the British, in one of the most spectacular airborne invasions in history.)

  • I hope I don't have to describe the successful oversea invasions of Mexico and Peru by the Spanish in the 16th century.

  • "In 1762 British Royal Navy sailors and marines succeed in taking the capitals of the Spanish West and East Indies: Havana in Cuba and Manila in the Philippines respectively." (Wikipedia, s.v. Amphibious warfare)

  • At the beginning of the 19th century, American-led forces successfully invaded the Barbary States. ("From the halls of Montezuma to the shores of Tripoli we fight our country's battles in the air, on land, and sea"...)

  • And anyway, in the second world war the Allied invasion of Normandy was the fourth successful seaborne invasion staged by the Allies, after the invasions of North Africa (operation Torch), of Sicily (operation Husky) and of Italy (operation Avalanche).

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  • $\begingroup$ Did you notice that all your examples are against your point? In every single one of them, ground troops were transported by ships. That means the actual battles were fought on solid ground by ground troops. The ships did little more than bring them to where thy were needed. $\endgroup$ – Burki Feb 11 at 11:48
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    $\begingroup$ @Burki: That's what an "amphibious assault" means. That's what seaborne or airborne troops do. I am answering a direct quotation from the question (at the top of the answer), not the entire issue. $\endgroup$ – AlexP Feb 11 at 11:56
  • $\begingroup$ yet, the OP seems to talk about navies and air forces actually fighting, not just transporting. And i assume that in his context, boh marine infantry and parachuting troops should be considered ground troops, because both do their actual fighting while on the ground. $\endgroup$ – Burki Feb 11 at 11:59
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    $\begingroup$ @Burki: The question explicitly says that invasions by embarked troops are problematic, verging on impossible. My answer provides a selection of successful invasions from the antiquity to the modern age. (And I don't understand why you would believe in all the exemplified invasions the invaders did not have to fight to be able to reach the destination and disimbark their troops... Some of them were just simple transportation, but others did involve naval combat. And anyway, examples of successful invasions where the invaders were not met at sea just show the important of naval defense.) $\endgroup$ – AlexP Feb 11 at 13:09
  • $\begingroup$ There was ls also the invasion of the Italian mainland at Anzio - it was stalled at the beach for a long time but ultimately was successful. $\endgroup$ – Paul Sinclair Feb 11 at 17:52

Navy + Air Force

Nobody likes to attack a fortress, because that's where all the enemy's guns and armor are located. A navy lets a military bring their fortresses to the battle front. Water can carry much bigger guns and much heavier armor than land can. That's why tanks only carry one 105 mm gun, but ships can carry 9-12 guns at 460 mm or more. An air force, on the other hand, allows a military to deliver fires quickly and deep within enemy territory. Ballistic guns generally max out at less than 50 km.

Land forces encountering an enemy fight a mostly 1-dimensional conflict along the front, with elevation providing a minor 2nd dimension, unless one combatant is able to out-maneuver the other by flanking/pincer/surrounding, in which case the battle is clearly 2-dimensional. Air forces guarantee that the conflict is always 2-dimensional, and that assets are vulnerable anywhere on the battlefield.

A space navy, on the other hand, combines both these aspects of a planetary navy and air force: they enable the military to "bring their fortresses to the front" as well as "project power over the entire surface". But a space navy only needs guns to deal with other space navies. As far as folks on the ground are concerned, the altitude alone is the threat. And this is why you can't use Terran history to extrapolate the effectiveness of space forces.


If genocide is your goal, because your society has decided that it can never co-exist peacefully with the enemy, then it is sufficient to bombard the enemy into oblivion. If you can tug big rocks around space, then "bombardment" is as simple as choosing a suitably large rock and dropping it on the planet. This is boring.

Most armies do not have genocide as their goal, because enemy civilians are potentially useful assets, along with the resources they hoard/harvest/mine. Getting those civilians to do what you want is pretty difficult with a navy or an air force, because the scale of firepower is inappropriate. If a farmer isn't growing crops for you, dropping a 2000 lb. JDAM on his head isn't going to achieve your goal. Holding some valued members of his family hostage, on the other hand, can be a pretty powerful motivator. For that, you need boots on the ground.

In fact, history is full of successfully invading armies taking over territory. From Alexander the Great and Genghis Khan to the founding of America to Iraq. Boots and guns have been very effective at projecting force onto a civilian population and subjugating other peoples. Sure, there will be resistance of varying levels depending the technology and resources available to both sides. But invaders which want to maximize assimilation rates will deploy propaganda and soft power to subvert and co-opt the resistance, in addition to the boots and guns which remind folks who holds all the cards.

Capital Invasion

If you have two evenly matched forces, then we expect any conflict to start in space, because each navy will try to interdict the other as it targets valuable assets. If the navies are competently commanded, then they may reach a stalemate and a detente. If one is overpowering, or simply executes better strategy, then eventually the other navy will succumb and eventually become ineffective.

At that point, what is stopping the dominant navy from flying straight to the capital planet or other major city worlds, and threatening the populations there with catastrophic bombardment? The aggressor simply needs to threaten to exterminate all life on the planet in order for the defender to unconditionally surrender and sue for peace. If there is any doubt about the ability or willingness of the aggressor to destroy targets on the valuable capital planet, they just need to pick an expendable target highly valued by the defender but otherwise useless to the aggressor, and obliterate it from orbit. Even if the planet has its own counter-orbital batteries, they cannot realistically defend against an entire navy which is free to choose its orbits and drop rocks wherever they like. And such batteries would obviously be the first targets for the invading navy. Even if the batteries could vaporize a meteor or two, it is always possible to pick a rock big enough that it cannot vaporize it before it impacts. Or, to drop more rocks simultaneously, from different approach vectors, than the defenses could possibly intercept. Note that the blockading navy can drop literal rocks. No need for fancy missiles or bombs or expending energy on beam weapons to target stuff on the ground.

Once the capital sues for peace, the aggressors just land their army "to enforce the terms of the treaty" and secure valuable shipments that are headed off-world, as well as keep a close eye on leaders who might have designs on running a resistance campaign. It should be expected that independent groups will run resistance efforts anyway, and thus, the army would be expected to secure the most valuable facilities and defend them against guerrilla attacks. Some resistance groups may be able to muster competent battalions of fighters and give the army something interesting to do, but the invading army would also be expected to have full air superiority at its disposal, not to mention orbital support for particularly intransigent defenders. Remember Vichy France.

Of course, a space-faring military would not even bother to think of separating Army and Air Force. Rather, it would think in terms of "interplanetary" and "planetside" forces. And that's why you would end up with just a Navy and an Army. And why the Army should be able to subdue any planet with an uncontested orbiting Navy.

Unlike some other answers, the Army would not just be responsible for "mop-up" operations. While it isn't going to project the majority of force (that comes from sitting at the top of the gravity well), it will project force most precisely and most intimately with the subjugated population. It is the force which will determine which citizens will become enemies or allies of the aggressor civilization. Sometimes, this may involve full-scale planetside battles, if the Capital planet breaks up into competing factions, and the nominal leading faction sues for peace, but the other factions decide to fight while holding on to valuable assets. Orbital bombardment is always a last resort, but in some cases, it may be necessary to fight full scale planetside battles to convince the resistance that their efforts are futile, and the aggressor has overwhelming force at all scales.

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IMHO when a space fleet of galactic power A defeats a space fleet of galactic power B in a space battle and then heads towards the planets of the sector it was defending, each planet will have to surrender to the space fleet of galactic power A as soon as it arrives within shooting range.

Each planet will have to surrender in order to avoid being totally devastated by the weapons of the space fleet from galactic power A. Thus planet after planet in that sector that had been defended by the defeated space fleet from galactic power B will have to surrender until and unless a superior fleet from galactic power B arrives to drive off the fleet from galactic power A. And no doubt the fleet from galactic power A will install weapons on each surrendered planet to totally exterminate its populations if they break their surrender agreements and revolt. Weapons like destroy-all-life-on-the-planet-bombs with computers programmed to explode if they detect revolt as well as dead man's switches to explode if the computers are deactivated.

So there won't be any invasions of individual planets. Instead space fleets will invade regions of space belonging to other space powers and force the planets to surrender.

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The problem I have with this question is that it assumes that invasions are necessary in order to win wars. Sometimes they are, sometimes they aren’t. It depends entirely on what the actual strategic objectives of the combatants are.

One of the big ones is to take control of useful resources. This may or may not include the entire population and/or economy of the target planet. Your assumptions regarding the viability of planetary invasions are reasonable if you only assume the intention is to take direct political and economic control of the entire planet, but there are lots of reasons not to bother going that far. You may only be interested in a very specific and limited part of the planetary surface. This might be a site of great political or religious importance (like Tokyo, Jerusalem, or Rome for example), or of economic importance like an area that’s extremely rich in a desirable resource.

While massive orbital firepower isn’t particularly useful in bringing a large population under your control, it’s very effective at carving out a space where you can operate with impunity and block any significant military action by the locals. The United States Military’s current situation in Iraq and Afghanistan is a perfect example of the former, while the more successful operations in Kosovo and Kuwait are an example of the latter. If you can get what you want while leaving ~90% of the surface of the planet alone, then you don’t NEED a massive invasion, you just need sufficient firepower in orbit to keep your invasion force protected while they do whatever you came there for.

If your objective is economic hegemony, then again you don’t necessarily need any boots on the ground at all. If the planetary economy of your target is heavily dependent on interstellar trade, then all you need is a good naval blockade to force them to the bargaining table in order to keep their trade lines open. This approach very nearly worked for the Germans in WW2, and arguably WOULD have worked if not for the truly colossal economic advantage the United States enjoyed. If Britain had been reliant on economies like those of South America as its lifeline, the U-Boats would likely have strangled the nation, if not into surrender, at least into military irrelevance vis-à-vis the German greater strategic concerns.

That leads to another of the most common strategic objectives in warfare, which is to render your opponent incapable of projecting force beyond their own borders. Historically achieving this objective often required occupying the enemy country in order to dismantle their military infrastructure, but that’s not even the case any longer today for most nations. Achieving air superiority over another country almost completely eliminates that country’s ability to project military force anywhere unless they have a nuclear weapons arsenal, and it’s arguably only a matter of time before defensive technology reaches a point where that will be possible for nuclear powers as well. In the science fiction context, this again involves a naval blockade followed up by targeted bombardment of the target planet’s military infrastructure. At that point you can either just leave and go about your business knowing they’re going to be a decade or two rebuilding, or leave sufficient military force in place to intercept and destroy anything that tries to leave the planet’s surface. The United States had achieved this kind of supremacy over Japan already before nuclear weapons were employed, and would have had no difficulty in strangling Japan into a third-world nation without ever having to resort to invasion at all. The choice between invasion and the use of nuclear weapons was strictly based on political concerns. Japan had already ceased to exist as a military threat at that point.

The bottom line is that you can’t talk about the feasibility of military actions like this in a vacuum. Military actions are always driven by strategic concerns, and you have to start from what the strategic objective is to determine what the feasible military solutions to achieve it are.

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  • $\begingroup$ there is also zero reasons to invade a planet for resources, to quote "invading a planet for resources is like Eskimo invading Central America to steal their ice". $\endgroup$ – John Feb 11 at 21:24
  • $\begingroup$ @John That's MOSTLY true, but not ENTIRELY true. You have to account for scenarios where there are resources that you just CAN'T get anywhere else, or that are so valuable that they're worth the effort of a localized invasion. You wouldn't invade Central America for ice, but CLEARLY you would invade the Middle East for Oil. $\endgroup$ – Morris The Cat Feb 12 at 13:41
  • $\begingroup$ but there really aren't any materials you can only get on habitable planets. by the time you can pose hte question your technology is advanced enough biological products are not exclusive to biology. $\endgroup$ – John Feb 13 at 1:43

Air forces or navies operating independently have only succeeded in causing surrender in a single historical case, that of Japan in WW2

This is simply not true.

NATO won the Kosovo War with air power alone. And while other examples of airpower alone are not so common, victories achieved largely by airpower are many, including the Gulf War. One points to the land forces at the end, but the war was over by that point and everyone knew it.

As you note, VJ day was due to airpower, but it really isn't. The US submarine force has decimated the Japanese shipping fleet and the country was falling apart. They were completely incapable of continuing the war effort to the point where they were arming the counter-invasion forces with spears. If you want to know just how badly it was going, read Saburō Sakai's biography - at the end he describes the complete inability for the crews to maintain their aircraft, and their unwillingness to do so as their morale was completely gone. And while I'll be blasted for stating this, it was clear the war was over and would have ended pre-invasion in any event. The military might have dreamed of a last hurrah, but by that point they weren't exactly held in the highest regard.

Naval examples are far easier to find, which I assume is simply because we've had boats longer than aircraft. The Battle of Lepanto ended Ottoman power in the Med, Yamen effective destroyed the Song Empire's ability to fight, Quiberon Bay ended any possibility for France in North America, the defeat of the Spanish Armada ended the possibility of invasion of England, as did Trafalgar. All of these battles outweighed any of the land battles of the same conflicts.

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  • $\begingroup$ and would have ended pre-invasion in any event. But when, and at what cost? You seem to have forgotten the fight for Saipan: boots on the ground were required; air and sea bombardment were futile. $\endgroup$ – RonJohn Feb 11 at 17:17

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