The emerging question on U.S. Iraq policy may be how much chaos is acceptable when U.S. troops finally leave. Over the last five years, U.S. expectations have devolved from removing the threat of nonexistent weapons of mass destruction, to establishing a pro-U.S. democracy, to “regime change,” and finally to containing sectarian violence. The final threshold for defining success may be leaving the Iraqi government with the ability to remain sovereign and repel incursions from outside its borders.

When President Bush leaves office in eight months, troop levels will likely be what they were after he declared “mission accomplished” in late spring 2003.

It’s as if the president is planning to leave a brick on the gas pedal of the Iraq military adventure as he leaps from the truck in January. This would leave the new commander-in-chief, even if he is determined to withdraw troops, heavily engaged in Iraq through half of his first term. This is not an acceptable transition of power.

Since Saddam Hussein was toppled, the assumption has been that the U.S. exit would come amid some sort of peace and stability period in Iraq. But what if that is not possible? What, then, are the terms by which the U.S. assesses its window for withdrawing?

In September, Mr. Bush said: “The principle guiding my decisions on troop levels in Iraq is return on success. The more successful we are, the more American troops can return home.” The problem is that the president stops short of offering a practical definition of success.

It may be more realistic to reverse the telescope and define what failure would be – say, for example, a Turkish invasion in the north, or overt support of Shiite forces by Iran. If the U.S. can be reasonably assured that this nightmare can be avoided, perhaps troop withdrawal can begin. Maintaining a military presence in the Persian Gulf and the Mediterranean, and perhaps even leaving troops on Iraq’s borders, may be a fall-back position.

Sen. Richard Durbin, D-Illinois, who supported funding the troop surge a year ago, this week expressed waning patience with the Bush Iraq policy, especially within the context of the president’s short-term tenure in the White House. With a $100 billion funding bill for military operations in Iraq and Afghanistan before Congress, Sen. Durbin said the president did not act in good faith last year when he argued for a temporary troop surge. Sen. Durbin said the president may not want to face a chaotic ending to the occupation of Iraq, but that chaos may be inevitable.

Managing that chaos may be the best the U.S. can expect.