IRAQ IS OVER. Iraq has not yet begun. These are two conclusions from the American debate about Iraq.

Iraq is over insofar as the American public has decided that most U.S. troops should leave. In a Gallup poll earlier this month, 71% favored “removing all U.S. troops from Iraq by April 1 of next year, except for a limited number that would be involved in counter-terrorism efforts.” CNN’s veteran political analyst, Bill Schneider, observes that in the latter years of the Vietnam War, the American public’s basic attitude could be summarized as “either win or get out.” He argues that it’s the same with Iraq. Most Americans have now concluded that the U.S. is not winning. So: Get out.

Because this is a democracy, their elected representatives are following where the people lead. Although the Democrats did not get the result they wanted in an all-night marathon on the floor of the Senate, from Tuesday to Wednesday this week, no one in Washington doubts that this is the way the wind blows. Publicly, there’s still a sharp split along party lines, but leading Republicans are already breaking ranks to float their own phased troop-reduction plans.

President Bush says he’s determined to give the commanding general in Iraq, David Petraeus, the troop levels he asks for when he reports back in September, and the White House may hold the line for now against a Democrat-controlled Congress. Leading Republican contenders for the presidency are still talking tough. However, the most outspoken protagonist of hanging in there to win in Iraq, John McCain, has seen his campaign nosedive. Even if the next president is a hard-line Republican, all the current Washington betting will be confounded if he does not, at the very least, rapidly reduce the number of U.S. troops in Iraq. After all, that’s what the American people plainly say they want.

The American people’s verdict is remarkably sharp on other aspects of the Iraq debacle. In a poll for CNN, 54% said the United States’ action in Iraq was not morally justified. In one for CBS, 51% endorsed the assessment — shared by most of the experts — that U.S. involvement in Iraq was creating more, not fewer, terrorists hostile to the United States. If once Americans were blind, they now can see. For all its plenitude of faith, this is a reality-based nation.

So Iraq is over. But Iraq has not yet begun. Not yet begun in terms of the consequences for Iraq itself, the Middle East, the United States’ own foreign policy and its reputation in the world. The most probable consequence of rapid U.S. withdrawal from Iraq in its present condition is a further bloodbath, with even larger refugee flows and the effective dismemberment of the country. Already, about 2 million Iraqis have fled across the borders, and more than 2 million are internally displaced.

Now a pained and painstaking study from the Brookings Institution argues that what its authors call “soft partition” — the peaceful, voluntary transfer of an estimated 2 million to 5 million Iraqis into distinct Kurdish, Sunni and Shiite regions, under close U.S. military supervision — would be the lesser evil. The lesser evil, that is, assuming that all goes according to plan and that Americans are prepared to allow their troops to stay in sufficient numbers to accomplish that thankless job — two implausible assumptions. A greater evil is more likely.

In an article for the Web magazine Open Democracy, Middle East specialist Fred Halliday spells out some regional consequences. Besides the effective destruction of the Iraqi state, these include the revitalizing of militant Islamism and enhancement of the international appeal of the Al Qaeda brand; the eruption, for the first time in modern history, of internecine war between Sunni and Shiite, “a trend that reverberates in other states of mixed confessional composition”; the alienation of most sectors of Turkish politics from the West and the stimulation of authoritarian nationalism there; the strengthening of a nuclear-hungry Iran; and a new regional rivalry pitting the Islamic Republic of Iran and its allies, including Syria, Hezbollah and Hamas, against Saudi Arabia, Egypt and Jordan.

For the United States, the world is now, as a result of the Iraq war, a more dangerous place. At the end of 2002, what is sometimes tagged “Al Qaeda Central” in Afghanistan had been virtually destroyed, and there was no Al Qaeda in Iraq. In 2007, there is an Al Qaeda in Iraq, parts of the old Al Qaeda are creeping back into Afghanistan and there are Al Qaeda emulators spawning elsewhere, notably in Europe.

Osama bin Laden’s plan was to get the U.S. to overreact and overreach itself. With the invasion of Iraq, Bush fell slap-bang into that trap. The U.S. government’s own latest National Intelligence Estimate, released this week, suggests that Al Qaeda in Iraq is now among the most significant threats to the security of the American homeland.

The U.S. has probably not yet fully woken up to the appalling fact that, after a long period in which the first motto of its military was “no more Vietnams,” it faces another Vietnam. There are many important differences, but the basic result is similar: The mightiest military in the world fails to achieve its strategic goals and is, in the end, politically defeated by an economically and technologically inferior adversary.

Even if there are no scenes of helicopters evacuating Americans from the roof of the U.S. Embassy in Baghdad, there will surely be some totemic photographic image of national humiliation as the U.S. struggles to extract its troops.

Abu Ghraib and Guantanamo have done terrible damage to the U.S. reputation for being humane; this defeat will convince more people around the world that it is not even that powerful. And Bin Laden, still alive, will claim another victory over the death-fearing weaklings of the West.

In history, the most important consequences are often the unintended ones. We do not yet know the longer-term unintended consequences of Iraq. Maybe there is a silver lining hidden somewhere in this cloud. But as far as the human eye can see, the likely consequences of Iraq range from the bad to the catastrophic.

Looking back over a quarter of a century of chronicling current affairs, I cannot recall a more comprehensive and avoidable man-made disaster.