RIYADH, SAUDI ARABIA -- Midnight slipped through the Middle East, leaving vast armies poised for battle, families steeling themselves for the worst and governments on knife's edge, as a war they know will change things forever in this part of the world appears just a breath away.

As Jan. 15, the deadline set by the U.N. Security Council for Iraq to withdraw its forces from Kuwait, ended here with no sign of compliance from Baghdad, millions in the region went to bed resigned to a deadly nightmare many of them had expected would never happen.

"Up until a few minutes ago," one Saudi woman said, "we were saying nothing is going to happen. But now, it's a bit too late, I'm afraid."

Barring a last-minute diplomatic miracle, the U.S.-led multinational force gathered here in the Saudi desert is expected very shortly to move to eject Iraqi forces from Kuwait with the most sophisticated and deadly conventional weapons ever used in human warfare. The Iraqis have vowed to fight long and hard.

In big ways and small, preparations are under way here, however, to endure this crisis.

Saudi Arabia tightened security and warned potential pro-Iraqi saboteurs against terrorist strikes. In Syria, similar measures were taken as units of its special forces were moved from Lebanon to Damascus, Reuter reported.

Iranian President Ali Akbar Hashemi Rafsanjani, who has said his country will stay neutral in any hostilities, appealed to Islamic countries to stop the outbreak of a "catastrophic" war. "In our view, Moslems and the region will suffer a great deal if a war breaks out," he said.

In Saudi Arabia, air raid sirens were tested, chemical gas masks sold briskly and supermarkets quickly ran out of bottled water, candles and canned meat.

State-run television broadcast instructions on how to secure a room at home against deadly gas. Apparently to promote security here and instill fear on the other side, Saudi television also showed paratroops billowing down from the sky, U.S. jet fighters zeroing in on targets, and tanks doing mock battle with the enemy in the desert.

Many residents of Riyadh left to stay with relatives and friends in the southern, presumably safer, port city of Jiddah.

Reports from occupied Kuwait City said Iraqi troops were rounding up families, but the purpose was not known. Iraqis have placed explosives at highway bridges in and around the Kuwaiti capital, sources said, but the reports could not be verified.

Navy Lt. Cmdr. Nathan Jones, duty officer for the U.S. military information office here, took calls from the press throughout the night. He had the same answer to the same question: No sign of Iraq beginning to move out of Kuwait.

"It's been a quiet night," Jones said. -- Caryle Murphy U.S. Marine Supply Depot Prepares to Fuel a War WITH U.S. FORCES, Saudi Arabia U.S. Marines are constructing a vast supply depot at their forward-most deployment in Saudi Arabia, poised to nourish an assault against Iraqi forces in Kuwait.

Earthmoving equipment has dug scores of holes and raised protective earth barriers for a supply management center that will dispatch ammunition, food, fuel and medical supplies to fighting units.

The Direct Support Command began setting up shop in this featureless terrain just 10 days ago, however, leaving much to be done before any attack can be launched with the full measure of support this base is designed to provide.

Marine Lt. Col. Jay Vesely, 43, of Chicago, said that his base would be ready to start supplying combat troops whenever orders come. Some ammunition, rations and medical supplies already have piled up, he said, and could be moved forward at once.

Two days of extraordinary desert rains -- giving way to bright sunshine -- slowed work here, he acknowledged. Protective holes where Vesely's men sleep and work filled with water and had to be pumped out or bailed by hand. Vehicles once parked on a carpet of desert dust are now mired in mud.

"We didn't come to the desert expecting rain," said Lt. David Grannan, 26, of Bloomington, Ind., who runs the camp computer system.

Grannan's computer shop brought protective gear, including women's nylon stockings to act as filters, to prevent desert dust and sand from fouling the sensitive equipment. But for the moment he and his men have been busy laying sandbags to prevent backed-up rainwater from seeping into their tents.

With the onset of hostilities, combat forces fighting Iraqis to the north would receive ammunition from this base through a system that begins at Saudi ports, passes through here to a Direct Support Group and ends with delivery by Mobile Combat Service Support Detachments in the battle zone.

Chief Warrant Officer Cathy Mazerolli, of Vacaville, Calif., said her maintenance unit has suffered from manpower shortages because of budget reduction efforts before the gulf crisis erupted last August.

"Before all this happened, the big thing was reduction of forces, so when we got out here in the desert, we didn't have all the manpower we needed to have," she said.

Also important in her men's eyes, she added: They have received no mail since arriving in Saudi Arabia Dec. 9.