Americans must take a lesson from history and not believe that the terrorist threat has been exaggerated or will go away, Defense Secretary Donald H. Rumsfeld said Wednesday.

He described a new, more ruthless and lethal terrorist enemy, "with no territory to defend, no treaties to honor, that measures progress in terms of decades, not days."

"With this sort of enemy, we cannot afford — and indeed could not survive — another holiday from history," Rumsfeld said.

In remarks to students at the Air Force's war college at Maxwell-Gunter Air Force Base in Montgomery, Ala., Rumsfeld also acknowledged that the Iraq war, like other battles, "has not been a steady, smooth glide path."

He said the enemy is unpredictable and frequently adapts to changing conditions, such as when insurgents began to use roadside bombs and other explosive devices after convoys became better protected against traditional attacks. He has made similar concessions before.

His remarks came as sectarian violence continued to rage in Balad, north of Baghdad, prompting the U.S. military to send troops there to back up the Iraqi Army.

The ongoing attacks in and around the capitol city have pushed the death toll of American troops to nearly 70 this month, putting October on track to be the deadliest month for coalition forces since January 2005.

Rumsfeld's speech echoed statements he made in August , when he warned an American Legion convention in Salt Lake City that the U.S. must not repeat the World War II-era mistake of trying to appease the enemy, even as fascism took hold.

While Rumsfeld did not specifically cite Bush critics in the speech, irate Democrats took issue with any suggestion that opponents of the Iraq war could be likened to those who wanted to appease Adolf Hitler before World War II.

On Wednesday he referred to those who say terrorism is "somebody else's problem, or it will go away," and countered that America has no other choice but to go on the offensive. And he urged patience with the Iraqi and Afghan governments as they struggle to build their democracies.