I think the majority of Americans are isolationist at heart. As long as world events don't bother them, or they don't know about them, they really don't care. Sure, everyone's all hugging each other over 9/11, but if you polled the population over whether or not they'd send their children or themselves (depending on age group) overseas to fight, I doubt many would be willing.
Just taking a look at voter turnout should convince most people that Americans care little for what goes on outside the home. It is due to that indifference that politics has become what it is today with minority (as compared to the population) lobby groups wielding large influences at all levels of government. Americans in general just don't feel like getting off the sofa to be active in helping determine the future of the country. I mean, just look at the AARP, which enough political clout based on votes alone that it rivals big business.
So, in answer to the topic, Americans are already isolationist. Given an extended conflict with a large number of casualties, it would only be a matter of time before the population clamors for a recall of overseas troops. This is Suddam's greatest weapon in that if he can inflict a large number of casualties on invading troops via biological or nuclear means, Americans will eventually become sick of war and lose the will to fight. The U.S. incurred about a million casualties in World War II and the country pretty much stomached the pain. Vietnam resulted in about 200 thousand casualties with a lower average killed per day and still resulted in an entire movement.
There are reasons for the drop in enlistment over the years and the emphasis of the U.S. military of quality smart weapons over large numbers of dumb weapons.