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I've been thinking of a scenario in which the US wants to control the entire world, via time travel, but I'm not sure what time to travel to.

The US is generally considered to have the most powerful military in the world today, but it could certainly not stand up to every other country at once. However, if the same military were to appear in 100 BC they could easily do so, due to their advanced technology (planes, machine guns, etc).

The idea is that the entire modern USA and what is in it would be transported directly onto it's geographical location in the past. This would "overwrite" anything else currently there, AKA the Native Americans.

For various reasons, the population of the US is highly supportive of this war, like they were in 1943. Therefore there is no worry of internal revolt. Industries are also geared towards helping the war effort. The government has been planning this for 10 years, so they have stockpiled lots of ammo, materials, fuel, etc. Assume their technology is modern with the exception of the (one-use) time machine.

The goal is to conquer every nation in existence at that time. I'll consider nations to be political units with more than 50,000 people. I consider "conquered" to mean "an American in their capital, with the authority to make and enforce laws". If there is a better criteria for either of these let me know. I think it would be best if this goal was accomplished within 20 years.

The US won't be evil, and would eventually let the people become normal citizens after their previous government is destroyed. However, the nations it is conquering will presumably resist at first. After the war gets going, the other nations will band together against the US like they did against Germany in WW2, so we would need to fight off everybody at once.

So: How far back would they need to go?

I would guess they would travel to sometime between 1500 and 1800 AD but I don't know enough specific history to get closer. I'm looking for the time in which it would be likely to succeed, given realistic events.

Edit I see a few answers stating that any time before 1949 would work, due to nukes. I would like to clarify that the US still wants the countries to be livable and usable, so nukes would prevent this. Also, for a story that's not very interesting. So please don't involve nuclear weapons anywhere in an answer. Sorry for the change, I honestly forgot about nukes when writing this.

The time travel works in an alternate-universe / alternate-timeline way. So the modern US drops all past "dependencies" when it is transported to the (alternate) past. There aren't any paradoxes with who made the US, it makes a new timeline.

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  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    Feb 8, 2023 at 3:47
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    $\begingroup$ Any time the question involves time travel, it gets out of control very, very quickly. $\endgroup$ Feb 8, 2023 at 14:58
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    $\begingroup$ There can be no time. Period. Anything you can do, your enemy can do. America goes back in time, leaving China to continue 'in the present'. China develops their technology unopposed. There is no longer an America. China wins. China develops the same time travel technology. China goes back a few decades before the Americans are going to get there. China takes over the world before the Americans arrive, and the Chinese technology is far superior to and more advanced than the American tech. So America goes back in time, only to be defeated by superior Chinese tech that also went back in time.. $\endgroup$ Feb 8, 2023 at 15:39
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    $\begingroup$ So America is no longer in the future, and China is no longer in the future. That leaves the other countries, who have no interest in going back in time to conquer the world 'in the past', because they are now the only ones left 'in the future', and so they win in the end. $\endgroup$ Feb 8, 2023 at 15:42

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Ruling out nukes...

Optimistically, I would say pre-1940, maybe pre-1930. The main reason is the domination of air power.

Prior to the 1940s, no one was close to having viable jet technology. Guided missiles (in anything like their modern form) didn't exist. Countermeasures against modern guided missiles weren't even conceived of. Radar wasn't ready for practical use. And so on.

So on the defensive, the US airforces would be in the position where they could detect any incoming aircraft, shoot them down with guided missiles that the enemies couldn't defend against or else engage them with aircraft that could outrun them by a huge margin. And on the offensive, the US could fly in aircraft without being spotted (except maybe visually) against anti-aircraft guns aimed either visually or with crude technology or else against defending aircraft they massively outclass in every way.

In short, they would immediately dominate practically every scenario involving air forces - which would massively increase their overall military effectiveness in various ways. Does the enemy send out their navy? Well, every ship is vulnerable to the US air power. Does the enemy try to send out an overwhelming number of ground forces? The US can counter with attack helicopters, AC-130's, and various other options that the enemy would be unable to prevent from decimating them.

Dominating the skies makes a lot of things one-sided. That also includes being able to go in and destroy the enemy's production and logistics. Bomb every factory. Destroy every train yard. Etc. Thus eliminate their ability to muster any kind of fighting force.

There are likely significant advantages in other areas as well. Such as modern armor-piercing weapons would likely be much more effective against older armored tanks/vehicles than the enemy would be prepared for. And conversely older weapons against would likely be much less effective against modern armor materials (in comparison to the armor of their day).


However, defeating someone militarily doesn't mean conquering them in an effective and stable way. And just because you can put troops on the group to occupy a country, doesn't mean you'll be able to keep it forever or change it into what you want. There's a lot to be learned from just the past few decades of the US's involvement in the Middle East / Afghanistan / etc.

Hatred by the local population, rebels, guerilla warfare, sabotage, and so on. These kinds of things can't simply be solved via superior technology. At minimum the US would need to muster large forces of occupying troops. And if we're sticking to something like the modern US, then the number of possible occupying troops is limited. For instance, AlexP has a comment doing the math that "each American soldier will need to occupy and control about 5 square kilometers" So at some point they would most likely become stretched too thin. That doesn't look good for stability.

If it turns out that they just don't have the manpower to hold all of the conquered territory after a certain point, then can it really be called conquered? The US might find itself in a position where they've bombed everyone to the point of posing no military threat, but yet be unable to rule them.

So another answer to this question might be end up being: never.

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  • $\begingroup$ America dominated the air in Vietnam and they still lost. Your last line is the best answer. 'Never". $\endgroup$ Feb 8, 2023 at 2:34
  • $\begingroup$ @JustinThymetheSecond To be fair, I find it hard to believe that sending a 300+ million population U.S. back in time to when the world only had a population of 100 million wouldn't result in the expected conquest. My guess is that the real problem is that you can sustain, e.g., planes (and other fuel-consuming things) for so long. You really can't store an infinite amount of (e.g.) gasoline, even if the time-shifted U.S. disallowed anyone but the military using it. How long could a time-shifted U.S. build modern bullets? Not forever. $\endgroup$
    – JBH
    Feb 8, 2023 at 3:13
  • $\begingroup$ @JBH Here is the ultimate paradox. America only militarized after it entered the Great War. Before that, it had no army. Without the Great War, America would not have the military capability it has - it would not need it. If America dominated the world, there would be no Great War. no American military, and thus no way America could dominate the world. The time frame HAS to be AFTER the Great War, and even after World War 2. Without those, no American military. You can not have a chicken without the egg. You can not have the egg without the chicken. Now add The American Civil War. $\endgroup$ Feb 8, 2023 at 3:25
  • $\begingroup$ @JustinThymetheSecond That's assuming rules of time travel that are not required, much less necessary for the question. There are plenty of examples in SciFi of time travel that simply shrugs its shoulders and moves forward, ignoring the dependencies on the past, which have already brought about a condition that's entirely transportable. I honestly don't consider any issue of time travel to even be pertinent to the question. It's only relevant if the OP agrees that such a rule is in play. So, in the tradition of good worldbuilding... there is no paradox. $\endgroup$
    – JBH
    Feb 8, 2023 at 3:30
  • $\begingroup$ On the other hand, the conquerors could learn from history and do what the Romans did. Start small and conquer just Mexico. Make the Mexicans Americans. Use the expanded man force to conquer South America piece by piece, and make the South Americans, Americans. Etc. After all, the most Roman soldiers in the Roman army that Caesar used to conquer Gaul were not born in Rome, but rather second or third generation Romans from elsewhere. $\endgroup$
    – AlexP
    Feb 8, 2023 at 9:36
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Anytime absolutely before 1950 and most likely anytime afterwards.

Before 1950, only the USA has Atomic Weaponary = insta-win.

Even if we exclude Nukes, The US Navy and specifically Their Aircraft Carriers mean that they have absolute supremacy over the Waves and with modern attack aircraft, they have absolute supremacy over the Air.

Sure, you still need boots on the ground, but a squadron of F35s are going to ROFLStomp anything and everything except the latest Near-Peer adversaries (and even that is likely to be in America's favor) - Look at the 1991 Gulf War and Desert Storm - that was 4th Gen fighters against 3rd Gen Fighters and it wasn't close. The US Armor with the simple fact they had Night Vision and better Optics were smashing the Iraqi Tanks - Iraq's amored force was no joke - T72s against an M1 Abrams isn't a one-sided fight - they are pretty contemporary - Desert Storm is what every battle against every adversary would look like.

The further back you go, the more laughable it gets - Wooden ships? May as well save on the ammo and just ram the things. Cannons? They will be wondering what Witchcraft and sorcery you are using that made their entire Artillery section explode from a single cluster bomb run.

Even today - the USA Nuclear Carrier fleet could realistically launch an alpha strike against most of their immediate opposition (China, UK, Russia, France etc.) and would likely cripple their Naval capability - from there, it's a matter of locking down international shipping and Trade (Most of these countries don't do enough in-house manufacturing or the exploiting of Raw Materials) - from there, it's a simple matter of Logistics. The only realistic factor that would mean defeat (excluding a Nuclear War) is the Birth/Death rate. Even then though, the US could theoretically pull off a devastating enough first strike, then pick-off adversaries and force surrender and then absorb that Man power as it went.

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    $\begingroup$ In principle, you are almost right, and I upvoted the answer. (Except the part about the aircraft carriers being of much use against 21st century People's Republic.) In reality, the USA was beaten by Somalia and Afghanistan, countries which did not even have armies. The point being that military might is not enough, political will to conquer is also needed. $\endgroup$
    – AlexP
    Feb 7, 2023 at 23:43
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    $\begingroup$ @LukeB You didn't exclude nukes in your question, and using nukes does not render an entire nation unlivable (Japan's still with us...). If you're looking for a non-nuclear solution, there is no period of time in U.S. history where the U.S. could conquer the world. We don't have enough people or resources to do that without weapons of mass destruction. $\endgroup$
    – JBH
    Feb 7, 2023 at 23:44
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    $\begingroup$ @LukeB Two things. First, if the opposition does not have them, you don't need to drop them everywhere, just on the big concentrations. Second, I spent an afternoon at Trinity Crater. It's about double background radiation by now. Got time travel then that issue is solved. $\endgroup$
    – Boba Fit
    Feb 7, 2023 at 23:44
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    $\begingroup$ @LukeB the big point about nukes is that without them no one else has a deterrent that could stop them, the US with modern armaments would win hands down against the rest of the world with kit more than 50 years older in design, nukes would be the only thing that could stop them, hence nukes are important regardless of if you want the US using them themselves or not 🤗 go back to any time when others had nukes and regardless of how much better your conventional forces are you still can't guarantee a win. $\endgroup$
    – Pelinore
    Feb 7, 2023 at 23:45
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    $\begingroup$ @AlexP - you are right about the Political Will needed - but in OPs question they specifically stated that the US Population was highly supportive of the campaign, so I discounted that. $\endgroup$ Feb 7, 2023 at 23:50
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If they must, just after WWI

First, let's note that this is a bad idea that the citizens of the USA will not be in favour of as soon as they understand the consequences. Two of the most obvious consequences are:

  • All the people that are outside the USA will (to all practical purposes) vanish forever as soon as this plan is implemented. People will never see their parents, children, siblings, etc that live overseas ever again. While there are some insular communities that may be relatively unaffected by this, there are so many immigrants and families with members who have emigrated that this plan would definitely fail to achieve popular support.
  • The standard of living will drop catastrophically. Look at a list of the top 10 categories of US imports and you will see that the majority are manufactured goods and the key non-manufactured category is "minerals, fuel and oils". A less technologically advanced world simply cannot produce what the USA needs to import to keep its economy running. Latest generation computer chips produced in Taiwan and South Korea are probably the most extreme examples, but there are a myriad of niche components that will be simply irreplaceable, not to mention medicines that are not produced in the US. Stuff across the USA will stop working, businesses will fail and people will die, all in very large numbers.

So, the USA has to take over a world that is as technologically advanced as possible to minimise the pain of bootstrapping the rest of the world up to the required tech level to support them, with as little will to resist as possible, with as many natural resources remaining as possible. Ideally it will also have centralised political structures that can be taken over from the top down to avoid having to personally go and conquer every little country personally.

The period immediately after the First World War is the best option. Europe and much of the rest of the world is exhausted from the war and, without intervention, about to be hammered with an influenza pandemic that killed more people than the war did. The great European empires are still intact, which means that taking over a handful of exhausted European nations gives control of much of the entire world by proxy. Literacy and scientific literacy levels are high enough that upskilling can occur swiftly, and key products like crude oil can be supplied immediately, albeit probably not in the required quantities. However, the modern USA has such an enormous technology advantage (including doctrine for use of these weapons) that they can trivially defeat any remaining military forces in a conventional conflict.

So, best option is to produce enormous reserves of strain-appropriate influenza vaccine and PPE and go back to just after the Armistice. (Remember that there will be lots of American GI's waiting to come home - who will never see their families again but can meet their would-have-been descendants.) Use a combination of military intimidation and offers of protection from the influenza pandemic to get as many countries to submit as possible, and it might be relatively easy to stomp on the ones that do not come on board once the pandemic has weakened them.

It sounds nice in theory, but remember that the USA has famously failed to win in Vietnam, Afghanistan and other locations regardless of their technological advantages. Despite your brash assertion that "The US won't be evil", its actions will cause enormous suffering and create a vast number of enemies, both foreign and domestic.

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  • $\begingroup$ Given that America is entirely dependent on the immigrant population (Silicon Valley, MIT, and Harvard would not currently survive without it,) there is a VERY big problem with sending what is essentially an immigrant-driven economy back in time to, well, destroy the countries the immigrants came from. $\endgroup$ Feb 8, 2023 at 2:39
  • $\begingroup$ @JustinThymetheSecond I'm assuming that there are no grandfather paradoxes involved, that the "shifted" USA is replacing the "past" USA in another timeline. There's no way this can work if this change is supposed to allow the US to develop the way it "did". $\endgroup$ Feb 8, 2023 at 3:00
  • $\begingroup$ Here's the thing. America developed the nuclear bomb in response to Japan bombing Pearl Harbor. If Japan and Germany had been under American 'domination', there would be no world wars, and thus no need for the nuclear bomb. No first Great War, no American militarization. America did not even have an army until it was forced to recruit one by its entry into the Great War. Same with every other American modern weapon. How can 'modern' America be explained absent the entire world history that led up to America? If America controlled the world, there would be no need for American military. $\endgroup$ Feb 8, 2023 at 3:18
  • $\begingroup$ @JustinThymetheSecond the same consideration applies whether it's a single CIA agent from 2023 being sent back in time or the entire continental (?) United States. If the time traveller goes to a different timeline then they still "know" their own history, if they are in the same timeline they will self-delete as soon as they make enough changes that the circumstances that led to them existing and/or travelling back in time no longer exist. $\endgroup$ Feb 8, 2023 at 3:37
  • $\begingroup$ The problem with sending an entire COUNTRY is the 'infinite loop' bit. Eventually, the civilization that was sent back will progress through time to the date that they all went back in time. They never progress beyond that date into the future. Every time they go back in time, they 'enrich' themselves, and their technology continues to develop. When they reach the date that they go back in time, their technology is all that much better. Then they can do more 'back in time'. And the loop keeps going around and around. When technology goes back in time, there is no way around this. $\endgroup$ Feb 8, 2023 at 14:56

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