President Bush's new strategic doctrine says that while the US will seek to enlist the support of the international community for its policies, America will not hesitate to act alone if necessary to exercise its right of self defense. Many of America's allies say that they resent the excessive unilateralism of the Bush Administration's foreign policy, but even President Clinton argued that America must be prepared to go it alone when no alternative exists. So the debate about unilateralism vs. multilateralism has been greatly oversimplified.
International rules bind the US and limit America's freedom of action, but they also serve American interests by binding others to observable rules and norms as well. Moreover, opportunities for foreigners to raise their voice and influence American policies constitute an important incentive for being part of an alliance with the US. America's membership in a web of multilateral institutions ranging from the UN to NATO may reduce US autonomy, but seen in the light of a constitutional bargain, the multilateral ingredient of America's current preeminence is a key to its longevity, because it reduces the incentives for constructing alliances against the US.
Multilateralism, however, is a matter of degree, and not all multilateral arrangements are good. Like other countries, US should occasionally use unilateral tactics. So how to choose when and where?
No country can rule out unilateral action in cases that involve its very survival. Self defense is permitted under Article 51 of the UN Charter, and pre-emptive self defense may be necessary when a terrorist organization presents a strong or imminent threat. President Bush's military action in Afghanistan was largely unilateral, but was carried out against a backdrop of support from NATO allies and UN resolutions.