The war in Iraq and the wider war against extremists in the Middle East is the defining conflict of the generation, and one the United States and its allies cannot afford to walk away from, the top U.S. commander for the Middle East told a Harvard University forum Friday.

Gen. John Abizaid, likened the 21st century battle against extremists to the great ideological and political clashes of the 20th century.

"Think of it as a chance to confront fascism in 1920, if we had only had the guts to do it," Abizaid said.

As difficult as the war in Iraq is at the moment, Abizaid said United States has little choice but to try to find a way to help Iraqis defeat insurgents and quell increasing sectarian violence and private militias in the country.

"It's too soon to say we have failed," he said. "We can't keep talking about it as if it's a disaster or a failure."

Despite the sobering realities, Abizaid said there is reason to believe that the situation in Iraq will ultimately stabilize. He said extremists in the region, including Usama bin Laden and Al Qaeda, have yet to go "mainstream."

But he said the challenges are daunting, including moving the Arab-Israeli peace process forward and containing Iran's "destabilizing role in the region." In Iraq, he said, there must be progress against sectarian violence in the next six months.

He acknowledged that military action is only part of an overall solution and that force alone can never bring about a stable Iraq. The best the military can do in places like Iraq and Afghanistan is buy enough time to allow local, stable institutions of power to take root.

He also suggested that soldiers on the front lines are more hopeful of a good outcome than the American public is according to some polls, a disconnect he blamed in part on the "24-7 news cycle" that he said tends to highlight the latest bombings or kidnappings.

"If I think it was hopeless I would tell you," he said. "It's hard. It's tough. We can do this."

Abizaid made his comments at a forum at Harvard University's John F. Kennedy School of Government. Abizaid received a masters degree in Middle East Affairs from the university in 1981.

His appearance came just two days after Abizaid told the Senate Armed Services Committee that announcing a timetable for starting a pullout of the 152,000 U.S. troops in Iraq would limit the flexibility he needs to manage the military transition there.

He also said the Army and Marine Corps are not big enough to sustain a substantial increase in Iraq, although he said adding 20,000 troops for a short period was possible.

Abizaid also warned of increased sectarian violence should U.S. troops begin to withdraw in four to six months, as proposed by some Democrats. It also would undermine U.S. efforts to increase Iraqis' confidence that their own government is capable of assuring their security, he suggested.