True, but the US soil really doesn't need much defending. There are large oceans to the East and West. To the North is an allied country, and to the South is a country with close economic and cultural ties. There are no countries that could feasibly carry out a transoceanic invasion of the US, even were they to exert the entirety of their military might.

Countries like Poland and South Korea are within the reach of geopolitical rivals, though. So places like those are where the US is garrisoning much of its combat ready troops. Which makes perfect sense.

I'm also not so sure what you meant by "It would seem natural to think your own soil is the only thing you can ever rightfully defend." Why wouldn't it be rightful to defend the territorial integrity of other countries? In fact, the US is obligated to defend it as per international treaties like NATO.