Beyond the issue of whether or when they achieve their goals in this war, U.S. commanders in Iraq will also be measured by a very human, and constantly changing, scorecard - the number of dead and wounded.
Defense Department statistics show that casualty rates were remarkably steady for much of the 20th century but dropped sharply in the nation's most recent conflicts.
In World War I about one in 15 U.S. troops was killed or wounded; in World War II it was one in 14. The rate climbed to one in 12 in Korea and fell back to one in 16 during Vietnam.
The rates have been going down ever since, with only one soldier in 760 killed or wounded in the first Persian Gulf war. There were no U.S. combat fatalities in Bosnia or Kosovo and to date only 66 in Afghanistan, including two killed Saturday. The Defense Department has not released statistics on the numbers of wounded in those conflicts.
"While any death is one too many, the fact is that the casualty rate for the recent wars are really low and the numbers in this war so far are tiny," says Thomas Mockaitis, a professor of military history at DePaul University.
Early in Iraq conflict
In a conflict not yet 2 weeks old, experts caution that it is far too early to draw any conclusions about how the numbers will add up. As of yesterday, there were 46 U.S. and 27 British soldiers killed. The Defense Department hasn't released updated figures for wounded, but an estimated 200 military personnel have been treated at U.S. and overseas hospitals, translating into an allied casualty rate of one out of every 1,100.
Casualty rates depend on a variety of factors, ranging from the length of the war to the terrain where it's fought. The military technology available, the nature of the mission and the amount of force used also are key factors.
The trench warfare of World War I resulted in staggering losses: 54,000 British killed in 10 hours at the Battle of the Somme. The fierce close combat on Iwo Jima during World War II left 25,851 casualties among the U.S. Marines. And some assignments are particularly risky: nine out of 10 World War II German submariners were killed.
Casualties, defined as any wound that incapacitates a soldier, have declined as more precise weapons have been produced that reduce the enemy's firepower, better body armor has been developed, fewer large-scale battles are fought and medical advances have reduced illness in military camps.
Experts say that the numbers of dead and wounded have dropped in the nation's most recent conflicts because they've been fought largely with air power and were more limited missions than the defeat of Nazi Germany - or an entrenched Iraqi regime.
"We really stood at a distance from the enemy when we fought those recent wars," says Thomas Keaney, a retired Air Force colonel who is executive director of the Foreign Policy Institute at Johns Hopkins School of Advanced International Studies.
In modern warfare, smart bombs fired from long distances reduce the number of bombing missions and the number of ground assaults necessary for the infantry, which traditionally suffers the largest share of combat casualties. In the 1991 Persian Gulf war, 39 days of bombing preceded a brief ground assault. Ground troops in Kosovo and Afghanistan also were assisted by major aerial assaults.
So far, the U.S.-led forces have hit Iraq with an estimated 8,000 precision bombs, including two 4,700-pound, satellite-guided "bunker buster" bombs dropped by stealth bombers Friday on Baghdad.
"What you have is less and less of a human element on the front lines," says Mark Parillo, a professor of military history at Kansas State University. "Armies have fewer and fewer people literally carrying rifles, at what they're now calling the tip of the spear, because technology has interceded."
The superior firepower has meant an enemy reluctant to face U.S. troops head-on. That means more skirmishes and guerrilla tactics and fewer large-scale battles.
"They know if they fight a conventional battle, on a battlefield, they will lose," says John Pike, director of Globalsecurity.org. "That's why the Viet Cong gave the U.S. such a hard time in Vietnam and why you have these guys using guerrilla tactics in Iraq."
Along with laser-guided bombs, global positioning systems to track targets and night-vision technology, soldiers are better protected by what they wear.
Dr. Clifford Cloonan, head of military and emergency medicine at a Bethesda medical school for the military, says that since World War II, Kevlar helmets, protective vests and ceramic plates, now standard issue for soldiers and marines, have done just as much to increase combat survival rates as having surgeons close to combat.
"We've gotten really good at protecting our soldiers," says Cloonan, of the Uniformed Services University of the Health Sciences.