WASHINGTON -- In a flashy debut for its biggest non-nuclear bomb, the Air Force on Tuesday dropped a 21,000-pound behemoth onto a test range in Florida, hoping the test would rattle nerves in Iraq as well.

The bomb test was declared a success, but movement on other fronts in the U.S.-led push toward war was murkier.

At the United Nations, the United States and Britain faced the prospect of defeat for their resolution giving Iraq until Monday to disarm or be invaded, and it appeared they might agree to a short extension of the deadline.

Defense Secretary Donald H. Rumsfeld suggested that even the participation of America's closest ally, Britain, in the combat phase of disarming Iraq was in doubt.

Until the wording of the resolution is finalized, "we won't know the answer as to what their role will be," Rumsfeld said of the British military, which is deploying 45,000 troops to the Gulf.

"And to the extent they are able to participate -- in the event that the president decides to use force -- that would obviously be welcomed," he added. "To the extent they're not, there are work-arounds and they would not be involved, at least in that phase of it."

Asked whether that meant the United States was considering going to war without Britain, he said, "That is an issue that the president will be addressing in the days ahead, one would assume."

At a joint news conference with Rumsfeld, Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said the number of American forces now exceeded 225,000 and more were en route.

Neither Myers nor Rumsfeld would say whether the 21,000-pound Air Force bomb that was tested for the first time Tuesday would be used in a war against Iraq. It is officially designated the Massive Ordnance Air Blast, or MOAB, although it has come to be called unofficially the Mother of All Bombs, a rough allusion to Saddam Hussein's claim before the 1991 Gulf War that that conflict would be the "mother of all battles."

"Anything we have in the arsenal, anything that's in almost any stage of development, could be used," Myers said.

Rumsfeld indicated that the big bomb, which was dropped out the back of a C-130 transport plane over a test range at Eglin Air Force Base, Fla., was as much a psychological tool as any weapon.

"The goal is to not have a war," he said. "The goal is to have the pressure be so great that Saddam Hussein cooperates. Short of that ... the goal is to have the capabilities of the coalition so clear and so obvious that there is an enormous disincentive for the Iraqi military to fight against the coalition and there's an enormous incentive for Saddam Hussein to leave and spare the world a conflict."