It is an epic and unprecedented decision that could transform the Middle East or result in a disaster that could come from a dozen different directions.

"The administration is throwing an immense, high-risk bet on this," said Bruce M. Russett, the head of Yale University's program for United Nations Studies. "If it doesn't work out, the rest of the world is not going to forgive us lightly."

At risk is not just the future of the Middle East, but the structure of alliances and conventions for international behavior that the United States helped design and defend since the defeat of Germany and Japan in World War II.

The unpleasant side effects and unintended consequences of Bush's policy are huge already. Anti-U.S. sentiment is soaring throughout Europe, as the United States squandered the moral authority and sympathy it gained as a result of the Sept 11 terrorist attacks. The United Nations teeters amid the administration's insistence that it has the right — even the duty — to act on its own.

Fear of the United States also is rising, and ripples of instability have spread, most prominently in North Korea. The isolated communist government is scrambling to build a nuclear arsenal, some experts say, because it fears that it may be the next member of the "axis of evil" targeted for attack. Iran, sharing that fear, is pursuing its own nuclear program.

Although Bush is sometimes accused of lacking what his father once dismissed as "the vision thing," his administration's vision is sometimes frighteningly clear.

Led by a handful of conservatives in the Pentagon and Vice President Dick Cheney's office, the administration envisions a post-Saddam Middle East where democracy is set, terrorism and Islamic radicalism shrivel and Israel and the Palestinians make peace.

The Bush administration's aims may be laudable and its cause in Iraq may be just. The opposition of France and an overwhelming majority of the international community, along with a minority of Americans, may be wrongheaded. Yet what disturbs many is Bush's belief that the United States is free to act as Saddam Hussein's judge, jury and executioner.

The debate over Iraq has opened deep fissures in the U.N. Security Council, exposed the North Atlantic Treaty Organization as an ineffectual bureaucracy and divided Europe between "old" and "new."

"We are witnessing the dismantling of everything that the wise men of the 1940s built," said Francois Heisbourg, a French defense analyst at the Foundation for Strategic Research in Paris, referring to the balance of power and the web of alliances that emerged after the defeat of Nazi Germany.

The terrorist attacks of Sept. 11 fundamentally altered the U.S. view of the world. Republican hawks seized on counter-terrorism as an organizing principle of U.S. foreign policy, and they distilled their views six months ago in a 35-page White House National Security Strategy calling for U.S. pre-emptive military action to fight terrorists and rogue regimes.

Advocates said the new strategy reflects the surging dangers in the modern world. Critics charged that it reflects U.S. contempt for global opinion, and that it may cost the United States dearly over time.

"It says to the world that we have a right, because of our might, to decide questions of war and peace without agreement from others. This is terrifying to other countries," said Dan Hamilton, the director of the Center for Trans-Atlantic Relations at Johns Hopkins University.

The architects of the U.S. strategy counter that international cooperation is oversold, tyrannical leaders and terrorist groups must be devastated and that the United States is a democratic actor in world affairs.

"To back down now, they'd be dancing in the terrorist camps. Their whole posture is based on the idea that the United States is weak," said Richard Perle, a prominent hawk and the chairman of the Defense Advisory Board, a Pentagon advisory group.

Yet some fellow Republicans and many Democrats voice dismay at the damage to U.S. relations with allies. The United States may find international cooperation drying up in such fields as counter-terrorism, rebuilding Afghanistan and Iraq and confronting North Korea.