Talk in Europe of a possible U.S. invasion of Iraq has been shifting lately. The panicked incredulity of a few months ago is turning into nervous resignation. Europeans increasingly consider an American invasion all but inevitable, whether they like it or not. And if the United States stubbornly insists on going forward, European officials privately acknowledge, their governments probably won't protest much. (The "European street" is another matter.)

More than that, there's some chance key European governments will participate in the military campaign: Italy by providing access to air bases, for instance, and Turkey by providing even more. And don't be surprised if British and French forces show up in the battle in or over Iraq. Not because they will have been persuaded that invasion is a good idea -- probably nothing will convince them of that -- but simply because they don't want to be left out. The only thing Europeans fear more than the United States invading Iraq is the United States invading Iraq alone, leaving the once-great powers of Europe standing impotently on the sidelines, unable either to stop America or to help it.

Now many Europeans are starting to ask a different set of questions: What about the day after the invasion? Does the United States have a workable plan for a post-Saddam Hussein Iraq? And, most important of all, does the United States itself plan on sticking around long enough to build a new Iraq that is reasonably stable, peaceful, and democratic? Or will the Americans bug out after a few months or a year, leaving the job of putting Iraq back together to the United Nations or to Europe or, perhaps, to Iran? These are legitimate questions. In fact, they're the right questions at the right time. If a war in Iraq is going to come early next year, as some administration officials have been hinting, then people on this side of the Atlantic might want to start asking such questions, too.

Does the Bush administration have the right answers? Maybe it does, but you really can't blame the Europeans for worrying. The foreign policy line of Bush's 2000 campaign treated "nation-building" and "peacekeeping" as dirty words. Today Bush articulates a more Trumanesque vision of the American global role after Sept. 11, but the old notion of a more limited American role abroad -- "Superpowers don't do windows" -- keeps incongruously popping up.

One gets a whiff of it in Bosnia, from which the Pentagon seemingly can't wait to extricate itself. And, more disturbingly, one sees it in Afghanistan, where the administration's aversion to nation-building and peacekeeping, and even to putting substantial numbers of troops on the ground to fight the war, is palpable. The Bush administration may have its reasons for limiting the U.S. commitment to Afghanistan, but the effect so far has been to cast doubt on American willingness to stay anywhere for the long haul, including in a post-Hussein Iraq.

But Iraq is no "window." It is a historical pivot. Whether a post-Hussein Iraq succeeds or fails will shape the course of Middle Eastern politics, and therefore world politics, both now and for the remainder of this century.

Europeans worry about that, and they're right to do so. If it's true that an invasion may be only six months off, this would be a good time to start thinking about D-Day plus 1. Not only Europeans but Americans, too, ought to know the kind of task they're about to undertake. For if the Bush administration is serious, then the United States is on the verge of making a huge commitment in Iraq and the Middle East, not unlike the commitment it made in Japan more than a half-century ago.

The idea then was not simply to get rid of a dangerously aggressive Imperial Japanese government, nor merely to deny the Japanese the capacity to launch another Pearl Harbor. It was to rebuild Japanese politics and society, roughly in the American image. American policy in Japan, as in Germany, was "nation-building" on a grand scale, and with no exit strategy. Almost six decades later there are still American troops on Japanese soil.

Iraq may not be that different. Surrounded as it is by vulnerable friends such as Turkey, by Arab states of tenuous legitimacy, such as Jordan and Saudi Arabia, and by such worrisome nations as Iran and Syria, Iraq's success after Hussein's fall will be a vital American interest if ever there was one. If the United States goes into Iraq, it better be ready to stay there for as long as it takes. When President Bush makes it clear to our European allies that he understands this, at least some of them may breathe a little easier. And so should we.

The writer, a senior associate at the Carnegie Endowment for International Peace, writes a monthly column for The Post.

Originally published in the Washington Post.