CAMBRIDGE, Massachusetts (Reuters) - The top U.S. general in the Middle East said on Friday that if the world does not find a way to stem the rise of Islamic militancy, it will face a third world war.

Army Gen. John Abizaid compared the rise of militant ideologies, such as the force driving al Qaeda, to the rise of fascism in Europe in the 1920s and 1930s that set the stage for World War Two.

"If we don't have guts enough to confront this ideology today, we'll go through World War Three tomorrow," Abizaid said in a speech titled "The Long War," at Harvard University's Kennedy School of Government in Cambridge, outside Boston.

If not stopped, Abizaid said extremists would be allowed to "gain an advantage, to gain a safe haven, to develop weapons of mass destruction, to develop a national place from which to operate. And I think that the dangers associated with that are just too great to comprehend."

Abizaid said the world faces three major hurdles in stabilizing the Middle East region: Easing Arab-Israeli tensions, stemming the spread of militant extremism, and dealing with
Iran, which Washington has accused of seeking to develop nuclear bombs.

"Where these three problems come together happens to come in a place known as
Iraq," said Abizaid, who earlier in the week warned Congress against seeking a timeline for withdrawing U.S. troops from the country that is wracked by insurgent and sectarian violence.

"The sacrifice that is necessary to stabilize Iraq, in my view, must be sustained in order for the region itself to become more resilient," Abizaid said.

A week after
President George W. Bush's Republicans took a drubbing in congressional elections largely because of voter anger over the Iraq war, Abizaid said the United States had underestimated the challenge of preparing Iraq security forces to stabilize the violent country.

"We thought we could go from U.S.-led to Iraqi-led without having to pay the price of the transition, in terms of manpower and resources, etc.," Abizaid said. "Now we realize we have to invest heavily in this transition so you can bring them up faster."

In testimony to congressional committees on Wednesday, Abizaid rejected calls to either boost U.S. troop levels to quell the violence or to start a phased withdrawal from Iraq.

He said the level of violence there was "unacceptably high" and said the 140,000 U.S. forces currently deployed there should focus on training Iraqi units.

Lawmakers from both parties criticized Abizaid's comments as showing the
Pentagon had not developed a new, effective plan for the Iraq situation.

Commentary: