Top secret Pentagon estimates in the hands of President Bush tell him to expect 3,000 to 30,000 Americans to die in the first 20 days of a war against Iraq. As many as 10,000 U.S. soldiers could be dead in the first week, the reports say.

The most realistic estimate, according to the Defense Department, is that 15,000 would die in the first three weeks. And there could be some days to rival the bloody battles of the Civil War. The record for U.S. casualties in any one day of fighting is held by the Battle of Antietam during the Civil War. There, on Sept. 17, 1862, 22,728 Americans were killed, wounded, captured or missing.

If the Pentagon's highest Persian Gulf estimate is correct -- 30,000 dead in the first 20 days -- that would be nearly half the number of Americans who died in the Vietnam War.

Those numbers have sobered President Bush and, until recently, caused him to beat the drums of war more softly than he did when he first ordered troops deployed to Saudi Arabia in August.

But, while Bush is appalled at the estimates, our White House sources say he is hardened by the fact that Iraq's President Saddam Hussein is banking on the low American tolerance for casualties.

According to U.S. intelligence, Saddam has his own casualty reports that make the American figures look like a skirmish. Saddam could expect 10 Iraqi deaths for every American soldier killed. He would not flinch at the loss of 10,000 of his men, but knows that the United States would balk at the loss of 1,000 of its soldiers in the early rounds.

Outspoken former Air Force chief of staff Michael J. Dugan said it best before he was fired for his candor: "The American people will support this operation until body bags come home."

U.S. mortuary units and thousands of body bags have been dispatched to the Persian Gulf along with two hospital ships and land-based medical units. The Pentagon is looking at high numbers of casualties, even if the Iraqis do not use chemical weapons. That is because there is no way to drive the Iraqis back without house-to-house fighting in Kuwait and perhaps Iraq.

There is no doubt that America would swiftly win any air battles, but the long Iraqi defensive lines are dug in with mine fields arrayed in front of them. Iraq favors entrenched defensive warfare, which helped them to beat back Iran for eight years. Dislodging Iraq from its heavily fortified positions would take infantry assaults, which are always costly.

As a rule of thumb, an attacking army should have a 3 to 1 advantage over its opponent in numbers of troops. But in this case Iraq would be on the defensive with a 4 to 1 advantage over the United States.

Arab forces allied with the United States would make up some of the difference. A combined Arab force of about 30,000 is holding down the front line at the Saudi-Kuwaiti border while U.S. troops are well behind. U.S. estimates suggest that the Arabs could hold out no more than four hours before falling back to the American lines.