In terms of protecting their borders the USA have one to the south with Mexico and one to the north (or east if you're Alaska) with Canada, neither of which seem like likely invaders. Contrast this to other large powers and you'll see they've fairly large borders to protect where invasions seem more likely. Despite this the USA has one of the highest military budgets as a percentage of GDP in the world.
Is this simply to back up a "bigger stick" diplomacy or are there invasion threats I haven't taken into account?
To be clear to those questioning why the comparison is initially around defending USA soil - it isn't that this is all an army is for, but it is definitely part of it. The assumption is that, all else being equal, if a country is in a safer, more defensible position, it wouldn't need to spend as much on defending against invasion. This would mean that its total military budget could be lower. It is just a metric to measure by.