We may now be witnessing the most radical reshaping of the Middle East since it acquired its modern form (and many of its modern problems) in the wake of World War I. What the British Empire began, the American Empire may be about to finish.

Most of us are compulsively pessimistic about the Middle East; too many "road maps" have led over cliffs. But this time there's a real chance it could be different. The overthrow of Saddam Hussein has been the mother of all wake-up calls. Unlike his predecessors, who thought peace could be brought by touchy-feely peace talks, Mr. Bush has grasped that military power is key: the magical spear that heals even as it wounds. By showing them just how easily Saddam could be overthrown, Mr. Bush has made it transparent to Iran, Syria and Saudi Arabia that Saddam's fate could befall them too.

I don't believe anyone in the Pentagon wants to stage another invasion soon; their hands are full. The aim is to put the frighteners on the region's Muslim powers. And it's working. When five Arab leaders met Mr. Bush on Tuesday, they pledged, with manifest penitence, that they would henceforth actively fight "the culture of extremism and violence." Not just al Qaeda: Hamas and Hezbollah too. And that is precisely why, to the astonishment of many, Ariel Sharon seems ready to make the concessions without which no peace is conceivable. For the first time in his life, he has acknowledged that Palestinians in the West Bank and Gaza have been under Israeli "occupation." He has pledged to "evacuate unauthorized outposts." And he has agreed to the creation of an independent Palestinian state with "territorial contiguity" (the week's key word). None of this would be happening if Mr. Bush had not established his credibility in the region by force.

So what's the catch? It lies in the paradoxical nature of American power. In 2000, Mr. Bush talked as if he wanted to diminish America's military presence overseas. But Sept. 11 led to a 180-degree turn in his thinking. His administration produced a National Security Strategy that stated an intention to extend the "benefits of freedom" to "every corner of the world," and asserted the right to pre-emptive military action against any threat to America's security.

Many critics have seized upon this "Bush doctrine" as a dangerous, even revolutionary departure from post-1945 U.S. practice. I am not so sure. For one thing, it is eminently desirable that free markets, the rule of law and democracy should be introduced in countries currently languishing under rogue regimes. For another, regime changes of the sort we have seen in Afghanistan and Iraq are an indispensable element of the war against terrorism. Terrorists are sustained by dictatorships and flourish in conditions of anarchy. The terrorist threat will never be contained if the U.S. does not eradicate breeding grounds. And a strategy of global containment is not really a major departure in policy.

The radical aspect of the doctrine is not the theory but the practice. When Mr. Bush says he is prepared to fight terror in "every corner of the world," he really can. And he really does. If this isn't imperial power, I don't know what is. But here's the paradox. Vast though America's military power has become, the idea that the U.S. has become an authentic empire remains entirely foreign to the majority of Americans, who uncritically accept what has long been the official line: that the U.S. just doesn't "do empire."

"America has never been an empire," Mr. Bush declared during his election campaign. "We may be the only great power in history that had the chance, and refused." Speaking on board the Abraham Lincoln, he echoed that: "Other nations in history have fought in foreign lands and remained to occupy and exploit. Americans, following a battle, want nothing more than to return home." Days earlier, Donald Rumsfeld had been asked by al-Jazeera if the U.S. was engaged in "empire-building in Iraq." "We don't seek empires," shot back Mr. Rumsfeld. "We're not imperialistic. We never have been."

The Victorian historian J.R. Seeley famously joked that the British had "conquered and peopled half the world in a fit of absence of mind." The Americans have gone one better. The greatest empire of modern times has come into existence without the American people even noticing. This is not absence of mind. It is mass myopia.

Unfortunately, this myopia is one of the things that makes the American empire very different from--and, I believe, less effective than--the last great Anglophone empire, the British one. Americans have no qualms about sending their troops to fight in faraway countries. But they expect wars to be short and the casualty list to be even shorter. Since the war in Iraq officially ended, 40 U.S. soldiers have lost their lives, some as a result of terrorist attacks. Already there is a queasiness about this. When can our boys come home?

The realistic answer is: not for at least five years, the minimum duration of occupation that will stabilize Iraq. And if the British experience of governing Iraq after World War I is anything to go by, 40 years might be more realistic. Alas, nobody in Washington is willing to contemplate a military presence on that time scale. The U.S. may be a "hyperpower," the most militarily powerful empire in history. But it is an empire in denial, a colossus with an attention deficiency disorder. That is potentially very dangerous.

I began on a note of optimism, pointing out just how much has been achieved by the war against Iraq. If Saddam's overthrow marks the beginning of a sustained attempt to build peace in the Middle East, we will have cause to celebrate the advent of this American empire. But if Iraq is just another ephemeral military adventure, then I am filled with foreboding. For the moment America loses interest in what it has initiated, the cycle of terror will resume.