I've addressed this theme several times on ET, most recently while discussing US policy in Iraq and what I believed to be the lack of alternatives to confrontation.
I believe a retreat from responsibility by the US would most likely lead to advancement into the resultant power vacuum by competing regional powers and stateless organizations, many possessing weapons of mass destruction, many led by groups and individuals who possess illimitable ambitions and little moral restraint, with no demonstrably effective international force to oppose them. The likelihood of regional wars and of one of the world's nuclear powers undertaking an extreme reaction to one or another provocation, or on the basis of some convenient pretext, would rise immeasurably.
It's possible that reaction to the use of nuclear weapons or other WMDs would be met by renewed efforts to ban them, but it's also possible that the post-Nagasaki taboo would be broken, leading to repeated use and eventually to acceptance of them as "normal" military tools. There is also no reason to believe that weapons development in the world has suddenly come to an end, and a more dangerous world would very likely give new incentive to proceed with a new generation of biological, chemical, environmental, information, space-based and other unconventional weapons.
Even with strong US involvement around the world, the global situation could still deteriorate along the above lines, but responsible use and projection of pre-eminent US power currently gives the best chance during the critical near term period to induce potential competitors to focus on economic development and peaceful co-existence. In the longer term, stronger transnational institutions would be desirable, but the United Nations has shown itself far from being able to assume such responsibilities. At this historical moment, the US remains the essential nation in the world both in terms of security and economics, and most responsible national leaders recognize that fact, however begrudgingly.
