With more and more U.S. troops dying in Iraq, emotions on the American home front are increasingly conflicted. Is this a breakthrough moment, when public sentiment shifts dramatically against a conflict, as it did during the latter part of the Vietnam War? Or is it just a low point in a war that ultimately the country will be proud to have waged?

"When Americans see the war that is being fought as somehow connected to larger purposes, that makes the war and its sacrifices more palatable," said Andrew J. Bacevich, a Vietnam veteran and professor of international relations at Boston University.

"If Americans begin to see that a war does not connect to larger purposes, then their willingness to sacrifice and continue supporting an administration declines. In short, we want our wars to mean something."

"World War II had lots of discouraging moments, but almost everyone saw that it had to be carried out to its conclusion," said historian Geoffrey C. Ward, whose 14 books include one on the Civil War.

"The difference here is that increasing numbers of people aren't sure it is worth it."

Vietnam may offer a better analogy, because the underlying argument for that conflict — the need for the United States to fight communist expansion — gradually gave way to a belief that the war was bogged down in a quagmire that was killing thousands of Americans a year. The public can rapidly lose faith in leaders if it does not think a conflict is winnable.

When public opinion tilted against the Vietnam War after the Tet offensive in 1968, President Johnson chose not to seek reelection.

Yet the comparison has limits.

There was a national draft during Vietnam that caused millions of parents to fear that their sons could be sent to war. That war also spawned a protest movement that seemed to aim much of its anger at U.S. forces. The Iraq war is being fought by an all-volunteer army, and most critics make a point of condemning the war, not the warriors.

Still, the same fears of a morass are slowing surfacing about Iraq, some historians say.

"I think we're looking at a watershed moment now, because this Iraq war stands in the shadow of the Vietnam War and all the failures we associate with it," said Robert Dallek, a biographer of Presidents Johnson and Kennedy.

"More and more people have the feeling that we're trapped in quicksand in Iraq, just as they did in Vietnam, and I don't see how Bush can regain enough credibility to say things aren't that bad," Dallek said. "He sounds like Johnson did, always saying there is light at the end of the tunnel, and the fact is people don't believe him."