John Mueller, a political scientist at Ohio State University, calls this casualty sensitivity "the Iraq syndrome." He described it in an influential journal article last year: "Casualty for casualty, support has declined far more quickly than it did during either the Korean War or the Vietnam War."

In the weeks after the U.S. invasion of Iraq in March 2003, public backing was powerful. But opinion began to shift quickly once the Iraqi army was beaten, its leader was forced into hiding, and chemical, biological or nuclear weapons were not found.

• By late 2003, public support for the occupation began to seesaw around 50 percent, according to Richard Eichenberg, a political scientist at Tufts University.

• In September 2005, 55 percent of Americans favored stronger efforts to withdraw because of the losses, a Gallup Poll found.

• Last October, 54 percent of registered voters believed the war wasn't worth the U.S. casualties or cost, a Hart-McInturff Poll found. In November, voters reversed the congressional balance of power in an election viewed as a referendum on Iraq.

Polling analysts believe Americans are more sensitive to casualties than in the past because they neither see vital interests at stake nor feel the "halo effect" from a clear prospect of success.

"When is it going to stop? We're losing a lot of youngsters," says former tanker Ed Collins, 82, of Hicksville, N.Y., who survived the assault on Normandy's beaches in World War II. "I went in when I was 18; that was young, too. But we fought for something. Now we have no idea who we're fighting for and what we're fighting for."

That's partly because the mission's focus has shifted repeatedly, the experts argue: from finding weapons of mass destruction, to deposing Saddam Hussein, to fighting terrorists.

When the number of Americans lost in Iraq recently passed the 2,973 killed in the Sept. 11, 2001, terrorist attacks, the parallel was noted by some. Some have also noted that Iraqi deaths far surpass those of the American military, with tens of thousands of Iraqi civilians killed in the violence.

Building a stable democracy in Iraq has been given as a justification for the war's sacrifices, and yet close to two-thirds of Americans think a stable, democratic government is unlikely to take hold in Iraq, according to a Dec. 8 poll by AP-Ipsos. Many believe Iraq has fallen into the chaos of civil war.

Americans instead tend to back wars to stop aggression, like the invasion of Kuwait before the first war with Iraq in 1991, polling indicates.