A four-figure number hovers 50 feet over a busy Philadelphia street, visible in an office window. It changes maybe once or twice a day like the cost of something.

A janitor once stopped, just to stare. "I see that number, and it makes me cry," he told Celeste Zappala, who keeps the running tally.

It is a number that strongly moves American opinion: the U.S. military's death toll in Iraq. Zappala's son, Sgt. Sherwood Baker, is one of the dead.

Other makeshift memorials rise up across the country as reminders of the war's human cost: flags planted in honor of the dead on the National Mall in Washington, symbolic tombstones at the Liberty Bell in Philadelphia, signs with fallen soldiers' names plastered to telephone polls outside Boston.

Americans may question this war for many reasons, but their doubts often find voice in the count of U.S. war deaths. An overwhelming majority — 84 percent — worry that the war is causing too many casualties, according to a September poll by the nonpartisan research group Public Agenda.

The country largely kept the faith during World War II, even as about 400,000 U.S. forces died — 20,000 just in the monthlong Battle of the Bulge. Before turning against the wars in Korea and Vietnam, Americans tolerated thousands more deaths than in Iraq.

Has something changed? Do Americans somehow place higher value on the lives of their soldiers now? Do they expect success at lower cost? Or do most simply dismiss this particular war as the wrong one — hard to understand and harder to win — and so not worth the losses?

The Associated Press recently posed these questions to scholars, veterans, activists, and other Americans. Their comments suggest that the public does express more pain over the deaths of this war.

A death toll of 3,000 simply sounds higher to Americans in this war than it did in other prolonged conflicts of the past century, for a number of reasons, the interviews suggest.

"As fewer Americans die in war, their loss is more keenly felt, not necessarily at a personal level, but at a collective and public level," says historian Michael Allen at North Carolina State University.

Jeffrey Greenwood, 17, of Plymouth, Mass., though unsure of the exact number of Iraq war deaths, says, "I know it's enough to make people angry."

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.

"If the public really believed that our war in Iraq now was about stopping aggression, stopping terrorism, then we would see a greater degree of tolerance for casualties," says Bruce Jentleson, a former policy planner in President Clinton's State Department who now teaches at Duke University.

Nancy Lessin, co-founder of the antiwar group Military Families Speak Out, says many people appear to believe that "one death is too many in a war that should never have happened."

At the same time, scholars suggest that America's instant technologies and its global power have conditioned its population to expect quick, painless results in almost any war.

"In a world of smart bombs and so on, you just expect the military to be able to insulate the military from getting killed — and to a large extent they have," says Christopher Gelpi, a casualty researcher at Duke University.

Precision air power helped the U.S. military succeed in the former Yugoslavia and the first war with Iraq, and scholars say that lowered the expectation of casualties in future wars. Improvements in body armor may have contributed to the same expectation.

Speed-of-light consumer conveniences, like cellular phones and digital cameras, also reinforce expectations of fast results that spill over into war, some scholars say. In what's called "the CNN effect," the unblinking eye of video news and unending chatter of the Internet quicken and maybe intensify the public's reaction to the carnage of battle.

"The American people have never been known for their patience, and I suppose with these 24-7 news cycles and access to the Internet, everything seems to have accelerated," says Richard Melanson, who teaches a class on public opinion and foreign policy at the U.S. military's National War College, in Washington, D.C.

America's young no longer feel personally threatened, either. The military draft is history. These days, mostly working-class teenagers volunteer to do the fighting.

Charles Moskos, a sociologist at Northwestern University, believes America has lost zeal for warfare because the children of its elite rarely serve. The all-volunteer military is one of many legacies of Vietnam today.

Bobby Blair, a Vietnam veteran from Holliston, Mass., recently spoke about Iraq to a church youth group. "None of them personally know of anyone who's in Iraq," he said. "They didn't realize how serious it was. I said, `Do you think we're watching a video game?' And some of them said it was almost that."

Greater wealth and smaller families make Americans even more protective of their children and more loath to send them into battle than they once were, some argue. They are "sort of hothouse kids," says Harvey Sapolsky, the retired head of security studies at the Massachusetts Institute of Technology, who notes, "My grandparents had seven kids, my parents had two."

Reassured by official optimism and quick success in the invasion phase, Americans never expected to lose so many of their young in this war. In the first weeks, 80 percent of the public thought the final U.S. toll would not surpass 1,000, a Gallup survey found.

The president addressed their disappointment when he declared at an October news conference: "The fact that the fighting is tough does not mean our efforts are not worth it."

But are Americans willing to hang in a tough fight anymore?

Some wonder if U.S. society, now populated by baby boomers who recall Vietnam and never knew the hardships of the Great Depression or World War II, has simply lost its stomach for great sacrifices. Or perhaps in a materialistic culture, priorities are simply elsewhere now. "Everybody's looking to get theirs," says Tony Bouza, a veteran and former Minneapolis police chief who wrote "The Decline and Fall of the American Empire."

Many analysts argue otherwise. Some say Americans would still abide far more troop deaths, as in the world wars, if the cause were clear and dear. Others say today such an attitude would only return in the event of an invasion of the United States.