Gen. David Petraeus long has had the difficult job of leading American forces in the Afghan War. Now he has been succeeded by a new U.S. commander, Gen. John Allen.

We appreciate Petraeus' service, and we don't envy Allen's position. He faces enormous challenges in Afghanistan in dealing with a dysfunctional nation and long-oppressed population, in difficult terrain occupied by elements seeking to prevent Afghan stability and simply wait out American patience.

Allen is not naive.

"There will be tough days ahead, and I have no illusions about the challenges we will face together," he said.

U.S. troops in Afghanistan face both open combat and guerrilla attacks. There are few well-defined fronts. There are obscure enemies and unspecified objectives.

Some Afghans are thankful that Americans are there, but others seek to reimpose radical Muslim rule.

Allen's work is cut out for him.