US foreign policy is in the midst of an unprecedented, but incomplete, diplomatic revolution. This transformation has resulted in new levels of cooperation in a worldwide war against terrorism. Yet it has also initiated a risky process that signifies a profound change in the nature of alliance systems, as well as a fundamental reassessment of relations with former Cold War enemies and friends. Sustaining a long-term US-Russian entente in the war on terrorism will require sustaining a positive Russian relationship with an expanding NATO, as well as with an expanding EU. The failure of US leadership to forge a deep and truly concerted relationship with NATO, the EU, and Russia could result in the degeneration of the global system into an increasingly violent anarchy in which both major and rising regional powers alike begin to take more assertive unilateral actions.