President Obama's willingness to take on enormous political risks is already almost commonplace. Ending the war, while unequivocally the right thing, is another one. After six years, it makes no difference whether U.S. troops leave in 16 months or 18. The risk for Obama and the challenge for the country lie in what we will do if -- some would say when -- serious violence erupts as U.S. troops depart.

The U.S. presence interrupted a struggle for political power that always follows removal of a government and eventually forced it into nonviolent channels. But the struggle is far from over. Recent political accommodations are extremely fragile, and it is likely that many angry groups have chosen to lie low until the Americans are gone.

Stable agreements to share power emerge only after the parties have tested each other's strength and will and their desire to fight has burned out. History shows that this takes many years, especially when all sides are heavily armed.

So the United States may face a departure in 2011 in the face of great instability. President Obama understands that could happen even if our troops were to stay five more years. There is no substitute for Iraqis sorting out their own political future. But after so much sacrifice and bloodshed, it may not feel much like a victory.

This article first appeared in the Washington Post.