Well that's the problem, isn't it? Recent wars have shown that the defender usually has a strong advantage. The US could keep its borders (and any allied countries) safe for a fraction of its current budget

Yes they could take care of all of their own needs for a lot less, but now they're taming care of Europes needs too. Thats the whole point, there would be a West Russia by now if the US only took care of its own borders. So the exorbitant budgets the citizens have to fund are benefitting average Europeans more than average Americans and of course the capitalist class above all of them. But the conclusion is undeniable, American workers suffering subsidizes the military which is now providing european security.