Presumably there are similar obligations wrt. NATO which are also influencing decisions?
Consider Articles 5 and 6: http://www.nato.int/cps/en/natohq/topics_110496.htm#
"At the drafting of Article 5 in the late 1940s, there was consensus on the principle of mutual assistance, but fundamental disagreement on the modalities of implementing this commitment.
The European participants wanted to ensure that the United States would automatically come to their assistance should one of the signatories come under attack; the United States did not want to make such a pledge and obtained that this be reflected in the wording of Article 5."
But it definitely wasn't intended as a loophole to allow the U.S. and Canada to weasel out from participating in a major land war with the Soviet Union. This would have been hard to do in any case, what with having two Army corps permanently deployed in Europe during the Cold War...
[0]In italics: The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.
The thinking is in turn that those troops therefore deter aggression because they act as their nation's guarantee of involvement. A similar principle was seen during the age of gunboat diplomacy, especially with the Pax Britannica.
The only reason the US keeps 30 000 troops so close to the DMZ in South Korea, is so that they won't be able to weasel out. Make no mistake, those boys and gals' are there as hostages.
If the DPRK decides to launch an attack, under pretty much any scenario those troops will be engaged in battle within 24h and I doubt that the US public would be okay with leaving them there to fend for themselves.
Read Tom Clancy's 1980s novel Red Storm Rising, which seemed pretty realistic to me. (Former U.S. Navy carrier officer here.)
Boomers were also the force of last resort in MAD.