What were the standards for U.S. military intervention during the 1990s? Why were these standards applied unevenly?
I think there was a period of confusion and redefinition of America's military role after the Cold War ended. It had gone on for so long that a large part of our military's mission and identity was wrapped and designed around opposition to the Soviet Union, and once that ended, this gigantic and very expensive array was used to invade or attack countries much less powerful than the Soviet Union, whenever supposed American interests were involved or threatened. I think that is why American foreign policy where the military was concerned seemed a little haphazard in the 1990s.
It is fascinating to examine US military intervention in this decade and to see how its various failures, notoriously Somalio, contributed initially to a kind of laissez-faire policy where the US did not necessarily see itself as the guardian of the world and focused on its own internal issues. Overtly, however, the US intervened in any conflict which impacted US interests, and also became involved in conflicts to prevent humanitarian outrages.
One of the major factors that seemed to influence US military intervention in the 1990s was the willingness of other nations to go along. Even though it was somewhat symbolic in terms of representation, many other countries were willing to sign on to the coalition in the first Iraq war and this helped the US to legitimize their massive military intervention. When support for pushing further into Iraq waned, the US also pulled back on their intentions.
Contrast this with the level of response in the Balkans when much of the EU was completely unwilling to commit any serious force, particularly on the ground, and so the US response was also limited in scope and in their willingness to put boots on the ground.
The standard for military intervention in the 1990s seemed to be that the US would intervene militarily in instances where US interests were at stake and/or where humanitarian needs dictated that we should intervene.
The first of these can be seen in the Gulf War of 1991. This was waged mainly to protect the world's oil supply, though it was partly about helping the people of Kuwait. The other of these can be seen in US interventions in places like Somalia and in the conflicts that occurred as Yugoslavia broke up. In those areas, the US intervened to try to prevent humanitarian disasters such as the "ethnic cleansing" that was going on in the Balkans.