Bob Shapiro sent along this paper:

Does unipolarity per se free the United States to use force abroad and thus make war more likely? Hardly. As the United States is learning, an imprudent war can still be costly for a sole superpower. The price tag for the Iraq occupation alone has been projected to exceed a trillion dollars. This is true not only in fact, as they say, but even more important, in theory. According to rational bargaining theories of war, an increase in the power of a hegemonic state should in itself have no effect on the likelihood of war. As long as all actors share common information about the change in power, the states losing relative power should simply give proportionately more concessions in disputes.

If so, the main effect of unipolarity on the likelihood of war, if any, should come from its effects on domestic politics and ideology, which could cause the expectations of the opponents to diverge. Under unipolarity, the immediate, self-evident costs and risks of war are more likely to seem manageable, especially for a hegemonic power like the U.S. that commands more military capacity than the rest of the world combined. This does not necessarily make the use of force cheap or wise, but it means that the costs and risks of the use of force are comparatively indirect, long-term, and thus highly subject to interpretation. This interpretive leeway may open the door to domestic political impulses that lead the hegemon to overreach its capabilities. If opponents sense that the hegemon is overplaying a weak hand, this increases the chance that the hegemon will need to fight hard to try to get its way.