The Monroe Doctrine requires the US to protect the Americas as a whole from foreign incursion. The US is obligated by treaty or law to defend Japan, South Korea, Taiwan, Canada, and practically all of Europe (NATO extends as far as parts of the former Soviet Union). Not all of these defensive arrangements are fully mutual, either in practice (the US has troops stationed to defend Germany but not vice-versa) or indeed in law (Japan has no obligations to the United States).
This is also far from the sole requirement that keeps costs up. Probably the more important factor is the incredible amount of cost expended to minimize friendly casualties. It's not enough to simply win a war, we have to win it very quickly and with very few casualties. China, as a counterexample, has no political need to make sure the war is wrapped up before the next election, nor any PR requirement to keep their own casualties exceptionally low. Instead, they can control their own media and--thanks to the one child policy--practically have a surplus of young men.
"Not all of these defensive arrangements are fully mutual, either in practice (the US has troops stationed to defend Germany but not vice-versa)"
For NATO, that's purely the way it gets executed. The only thing special about the USA in http://www.nato.int/cps/en/natolive/official_texts_17120.htm is that it is the place where the treaty is kept. Otherwise, the treaty is symmetrical. There are/were NATO (not only US, but also from other countries; for example, http://en.wikipedia.org/wiki/NATO_Air_Base_Geilenkirchen#Ope... shows there still are 12 other NATO forces in Germany) troops in Germany and not in the USA because that was the most likely front of World War 3.
The USA made tremendous efforts to help Europe in world war 2 and the Cold War, but it could have slowly decreased its effort once the western economies recovered, if it wanted to.
"It was understood, however, that Japan could not come to the defense of the United States because it was constitutionally forbidden to send armed forces overseas."
Given that this was forced on Japan by the USA and given the huge geopolitical influence he USA has, I would think they could have changed it, too.
The USA may have laws or morals that make it feel obliged to do more, but that are things it does to itself. I remain that the situation is (utterly simplifying and ignoring lots of facets):
- the USA polices the world, but cannot really afford to do so.
- large parts of the rest of the world keep financing the USA by ignoring that 'cannot afford' part. That keeps the dollar as a fairly strong currency.
And yes, the cost of surgical strikes can be way higher than that of a "win this war, whatever it takes" approach.
This is also far from the sole requirement that keeps costs up. Probably the more important factor is the incredible amount of cost expended to minimize friendly casualties. It's not enough to simply win a war, we have to win it very quickly and with very few casualties. China, as a counterexample, has no political need to make sure the war is wrapped up before the next election, nor any PR requirement to keep their own casualties exceptionally low. Instead, they can control their own media and--thanks to the one child policy--practically have a surplus of young men.