STRASBOURG, France Welcomed with thunderous cheers, President Barack Obama pledged today to repair damaged relations with Europe, saying the world came together following the 2001 terrorist attacks but then "we got sidetracked by Iraq."

"We must be honest with ourselves," Obama said. "In recent years, we've allowed our alliance to drift."

The new U.S. president said that despite the bitter feelings that were generated by Iraq, the United States and its allies must stand together because "al-Qaida is still a threat."

Speaking before a French and German audience at a town-hall style gathering, Obama also encouraged a skeptical Europe to support his revamped strategy for rooting out terrorism suspects in Afghanistan and Pakistan, and said Europe should not expect America to shoulder the burden of sending in combat troops by itself.

"This is a joint problem," Obama said on the cusp of the NATO summit. "And it requires a joint effort."

He opened his appearance with a 25-minute speech in which he set a dramatic, long-term goal of "a world without nuclear weapons." He said he would outline details of his nonproliferation proposal in a speech in Prague on Sunday.

"Even with the Cold War now over, the spread of nuclear weapons or the theft of nuclear material could lead to the extermination of any city on the planet," Obama said.