With 250,000 soldiers, sailors, airmen and Marines in place and more on the way, U.S. military commanders and their Kuwaiti allies have begun taking some of the final steps necessary to launch an invasion of Iraq.

Army Gen. Tommy R. Franks, chief of the U.S. Central Command, conferred with his team today at an ultramodern U.S. command center in Doha, Qatar, from where he would direct an assault on President Saddam Hussein's forces from land, air and sea. Some U.S. and British troops have moved to assembly areas in the Kuwaiti desert, closer to the Iraqi border. They have been issued ammunition and have said goodbye to hot meals. Contractors hired by the Kuwaiti government, meanwhile, have punched holes in an electrified fence and begun leveling earthen defenses along the border to make way for the armored columns that would head north.

To deflect any preemptive missile strike or later counterattack by Iraq, armored vehicles with chemical weapons detectors have been deployed in recent days in the streets of Kuwait City, about 25 miles southeast of here. And the U.N. border troops who guard the demilitarized zone between Kuwait and Iraq have begun evacuating civilian and nonessential military personnel.

The United Nations announced today it had withdrawn its personnel from observation posts on the Iraqi side of the zone. "All of them from the Iraqi side have been moved out," said Daljeet Bagga, spokesman for the U.N. force. "In case something happens, we do not want to be caught."

None of the moves necessarily signals imminent hostilities. But taken together they underscore that the invasion force is poised to move as soon as the diplomacy at U.N. headquarters in New York winds up. "If the president makes the decision, we're ready to go," said a senior U.S. military official, who spoke on condition of anonymity.

Still unsettled is what to do about a northern front in Iraq. Unless it is reversed, the recent Turkish parliament vote rejecting a major U.S. military deployment near Turkey's border with Iraq will preclude a full-fledged invasion by the 4th Infantry Division from the north as originally envisioned by war planners at the Pentagon.

"It becomes harder and it becomes slower," acknowledged the senior military official, who was interviewed recently at another base in the region. "It's so much easier with the Turks' cooperation. It's so much harder without it. But it can be done."

To compensate, U.S. commanders could quickly dispatch elements of the 101st Airborne Division or the 82nd Airborne Division in Kuwait to northern Iraq once war begins. But commanders are still considering sending a heavier mechanized unit across Iraq, bypassing Baghdad, to seize the north, while other units concentrate on the south and territory around the capital.

"You shouldn't rule out the possibility of making a land move from south to north," the official said. "It would take a while, but all options for getting to the north are on the table."

Such uncertainties are not unusual in the final days and weeks before a war or even after it begins, according to veteran officers.

"As soon as we pass the line of departure, the plan's going to change," said Lt. Col. George Smith, a top planner for the commander of the Marine forces, Lt. Gen. James T. Conway. Such flexibility enables Marines to outflank the enemy rather than stick to outdated designs, he said. "We're looking to operate faster than the enemy."

At Camp Commando, the main headquarters for the more than 55,000 Marines in Kuwait, the once tiny tent town has grown into a bustling city in the desert about 25 miles south of the Iraqi border. Humvees and trucks rumble down the dusty roads, while contractors work to expand the size of the encampment by knocking down the sand berms ringing it and building new ones farther out. A camp designed for 800 residents now hosts nearly 5,000, including hundreds of British troops.

"The initial footprint was very small compared to what we are today," said Lt. Col. Paul Lebidine, the deputy post commander. "And we're still growing."

Mornings begin with the pop-pop-pop sound from a nearby firing range where Marines test their M-16 automatic rifles; afternoons are marked by increasingly regular Scud drills in which Marines yank their gas masks over their faces and rush to the nearest concrete-and-sandbag bunker.

By contrast, military commanders have been surprised by the limited scale of preparations by Hussein's forces, as shown in intelligence reports.

The reports indicate Iraq has moved some surface-to-surface missiles closer to the southern border with Kuwait and shifted a Republican Guard division in the north toward Baghdad and Tikrit, Hussein's home town.

"We see some military movement but not as much as you would expect for a country that's under threat," the senior military official said. Hussein has "probably made the strategic decision that he'll give ground in the south -- and in the north if we get established in the north -- and he'll make a stand in central Iraq. That doesn't mean Fortress Baghdad necessarily, but at some point there will come a point where he fights back."

Glasser reported from Kuwait City.

Soldiers from the 44th Air Defense Artillery, attached to the 101st Airborne Division's 3rd brigade, practice wearing protective equipment in Kuwait.