What? You are absolutely denying ten thousand years of recorded human history if you are claiming that taking over another country provides no pay-off. There are resources, there is money, there are people to exploit, there is land, etc, etc, etc. If America decided to carpet bomb all of Kuwait, take it over, and steal all of Kuwait's oil reserves and begin producing, they have benefited. How would individual people in Kuwait be able to defend against a better prepared and large military that use force
The answer is that they cant, and that is the proof that anarchism fails as a reasonable goal for society. You could argue we have anarchy right now. You have absolute freedom to do what you wish to do, but under anarchy, there is no expectation that you will not receive violent consequences for not doing it. In anarchy, the majority or the most powerful take power and impose their will on others, and the individuals have no power to stop it other than to convince those in control to be reasonable, or work together and force each other to work against those that wish to do worse harm than your current government does. Anarchy leads to government.
What is anarchy's response to a military invasion? It is either to try and fight them off as individuals(which considering the fact that we don't have any major anarchist areas in the world right now except in relatively remote or unstable areas, proves pretty much doesnt work), let themselves get taken over and exploited, or join together, agree on imposing a justifiable and relatively small amount of violence and force on each other to create a standing army of their own, recognize common borders, and defend their "country."