There’s a compelling argument for why the United States can’t set a time line for an Iraq troop withdrawal.

But President Bush can’t make the case. It would require a degree of honesty about his failings in Iraq that he won’t muster. Instead, his pre-9/11 presidential speeches have offered more bromides and false optimism. “We’re carrying out a clear plan to ensure that a democratic Iraq succeeds,” Bush said – though the “plan” is failing and Iraq is in chaos.

Yet the president is right on one point: We can’t leave any time soon.

The Bush administration’s actions have created a dangerous new training ground for Mideast radicals in Iraq. Call it Sunnistan – three governorates including the infamous al-Anbar province, along with swaths of Baghdad. Sunnistan has become a bastion for jihadis and hard-line Baathists.

This needn’t have happened. As has been documented endlessly, the White House failed to plan for a Sunni insurgency and indeed accelerated its formation. U.S. troops were sent into Anbar without any counterinsurgency training; heavy-handed force angered Sunni tribesmen, who then turned against U.S. soldiers. This created the sea of alienation in which jihadis and Baathists now swim.

Belatedly, the White House woke up and tried to woo Sunnis into the political arena. The U.S. army now trains its troops in Iraqi culture and language and tries to win Sunni “hearts and minds.”

But U.S. efforts to uproot insurgents have so far failed – propelling critics to argue for near-term troop withdrawal. They say the U.S. troop presence fuels the insurgency and that a withdrawal would ease Sunni complaints. Iraqis would then be able to solve their problems themselves.

Not so.

The hard core of the Sunni insurgency won’t stop fighting if the Americans leave; it is out for power. Ex-Baathists dream that desperate Iraqis will welcome back a Sunni strongman. Radical Sunni Islamists despise Shiites as infidels and will fight to overthrow any government they lead. All sides are preparing for full-scale civil war if the Americans leave.

Followers of the radical Shiite cleric Muqtada al-Sadr are already talking of the immense bloodshed that would follow. They hope it will usher in a Shiite military victory (with Iranian help) and a Shiite religious state.

So forget the claim by some commentators that Iraq can simply split into separate Shiite, Sunni and Kurdish statelets. This is a pipe dream. There will be no peaceful separation.

However, a civil war won’t lead to victory for either side.

Instead it will likely turn into an interminable regional struggle, drawing in guns and money from Iraq’s Sunni and Shiite neighbors.

One of the likeliest scenarios is put forward by an Iraqi blogger by the name of the Mesopotamian in the afterword of Thomas Ricks’ new book: “Fiasco: The American Military Venture in Iraq”:

Anbar province, he writes, would fall to Sunni insurgents “even before the last American soldier leaves Baghdad.” Shiites and Sunnis would battle across Baghdad. Kurds would move to capture the key oil city of Kirkuk, leading to a Turkish invasion of Kurdistan. The Kurds and Shiites would turn to Iran for protection, and in response, a torrent of men and money would rush into Sunni areas from Syria, Jordan and Saudi Arabia.

“All join an infernal orgy of death and destruction,” the Mesopotamian continues, as oil prices skyrocket past $100 a barrel and “fanaticism sweeps the region.”

I would add that Iran’s protectorship would probably produce an even stronger form of religious rule in Shiite areas of Iraq. Meanwhile, Sunnistan would be training jihadi terrorists to blow up Saudi oil fields and to attack regimes in the Gulf states, Jordan and Egypt.

A senior U.S. military officer told me recently that if U.S. troops pull out of Anbar, they might soon have to return to retake Saudi Arabia. “The consequences of a pullout are worse even than the administration thinks,” he added.

That said, there is no guarantee that things will get better if U.S. troops stay.

Iraq’s new government is flailing and weak. The Iraqi forces we’ve trained are far from capable of fighting alone (so much for the Bush mantra that “we’ll stand down as they stand up”).

A leaked report from Col. Peter Devlin, intelligence chief of the U.S. Marine Corps in Iraq, concludes that the outlook in Anbar province is grim and won’t improve without a major infusion of U.S. aid and troops.

What President Bush has neglected to say is that Americans face a bitter choice produced by the mess the White House has made: pull out U.S. troops soon and face certain disaster, or leave them in to enforce a policy that is failing.

There is, of course, a third choice: Change a failed U.S. policy. Debate Col. Devlin’s report. Rethink what it would take, in money, men and time, to make Iraq more secure.

President Bush no longer has the credibility to lead that debate. Do Democrats have the guts?