I don't think it's possible to conquer the world for a significant period of time. Even if you built up a gigantic army, and you manage to destroy all governments in the world, I can guarantee you that either unrests or revolts against your rule would force you to give up some land, or due to the huge vast government you'd have to have, some of your subordinants are going to turn against you and you'll have a civil war and then the "empire" would split up into individual countries again.
Without future communications and transport technology (to make the government more efficient at crushing dissent) or mind-control (or mind-drugging) or genetic cloning technology to create an army of super clones that will always be loyal, you're stuck with working with humans, and humans will fall into groups and fight against one another. A LASTING "One World" government doesn't seem possible at this stage. You might be able to persuade all governments to unite, for maybe, oh say 5 years, but it'll fall apart. Just look at the UN - they aren't a world government, but if you see the arguments between nation states, you'll realize that conquering the world, no matter how powerful a conventional, modern day army you have, is not possible for the long run.
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