The fewer American soldiers in Iraq the better, but President Bush's decision to withdraw about 8,000 troops is a trade-off, really. The war in Afghanistan has become increasingly deadly, and more U.S. troops are needed there to combat attacks by - and contain the growing influence of - Taliban forces and other Islamic insurgents. So as U.S. soldiers leave Iraq, 4,500 will head to the other war, the one that should have commanded the administration's resources and focus from the start. Mr. Bush and his generals are trying to make up for lost time.

It was must be emphasized, however, that U.S. troop strength in Iraq will remain fairly stable, even with this drawdown, which will be carried out through February. Any further decisions will be made by the next occupant of the White House.

What makes this withdrawal possible at this juncture in the 6-year-old Iraq war is the success of the administration's troop surge and its complement of Iraqi Sunni fighters who joined the Americans to help fight al-Qaida insurgents and reduce the deplorable level of violence. Their role in improving security can't be underestimated, and they must be included in the equation as Iraqi armed forces assume more responsibility and the Shiite-dominated government struggles to build a democratic and safe Iraq.

But as Mr. Bush candidly explained yesterday, progress remains "fragile and reversible." Violence is down in areas where the U.S.-led surge is operating - way down. But the improvements are relative, and victory not easily measured or defined. Instead of thousands of Iraqi civilians dying monthly, a few hundred deaths are recorded. Suicide bombers ply their deadly trade less frequently, but they remain a factor in street life. Some neighborhoods may be returning to "normal" in one sense, but there are many areas in which longtime residents were forced from their homes in ethnic purges and displaced. Sunni-Shiite relations remain tense or worse. As Wayne White, a former State Department diplomat who worked with the Iraq Study Group, told participants at a Middle East Institute conference this week, "This is a bitter, traumatized society."

The Shiite-dominated government of Prime Minister Nouri al-Maliki has shown little enthusiasm for mending the sectarian divide, bringing the Sunni Arab fighters into the government's military or providing them with meaningful jobs. The concern among some American military leaders is that U.S. soldiers are keeping the lid on - and once they pull out, ethnic rivalries will flare again.

That's why progress on reconciliation is critical to maintaining improvements in security and safety. The Bush administration needs to reinforce that with the al-Maliki government and insist on a robust, sustained effort to breach the divides among the various groups. The Iraqi leadership shows little interest in uniting the country.