EASTERN SAUDI ARABIA -- The siren broke the silence of Friday's dawn with a plaintive wail at 4 minutes after 6 -- an alert that an Iraqi gas attack was on the way.

For the first time in three-quarters of a century, U.S. soldiers braced for an assault by chemical weapons. The several thousand Marines at this vast supply camp south of the Kuwaiti border piled out of their sleeping bags and reached for gas masks, protective suits, rubber boots and gloves.

The Marines had trained repeatedly for this moment -- how to fit the gas mask on snugly enough that no poisoned air seeps in around the edges, how to fasten down the heavy protective suit so liquid or gaseous agents cannot slip through the cracks, how to tie on the boots to prevent death from creeping up their legs.

But this time the alert was real -- MOPP IV, top priority, meaning there was fear Iraqi President Saddam Hussein had sent Scud missiles to rain chemical death on the troops.

Three hours earlier, he had fired a number of the missiles at Israel, armed with conventional high-explosive warheads. Another had been fired at Dhahran, Saudi Arabia, several hours' drive south of here, only to be destroyed in the air by a U.S. Patriot missile.

Now, I thought as I struggled from the fog of sleep, he has taken the next step. He has sent chemical weapons down on U.S. troops, carrying out the threat that has been implicit in all the calculations about peace or war since the Persian Gulf crisis began last August.

Lt. Pat Gibbons, a Marine press escort, was the first to get a grip on the situation. "Gas mask first," he said, and the tent began to shuffle with the sound of Marines and reporters unpacking canvas-and-rubber masks.

The tone of Gibbons's voice made him appear calm. Strangely, so was I. There no panic in our tent, only dread.

We strapped on gas masks and pulled on protective pants. But they had no belts, no drawstrings. Improvising, I pulled the belt from my own trousers and ran it through the loops in the protective gear.

In the predawn darkness, our movements were clumsy. My glasses fell somewhere in the sand; a companion's boot laces disappeared.

Outside the tent, the camp seemed deserted. Following training manuals, the Marines stayed inside the canvas shelters, sweating in the weighty gear despite the pre-dawn chill.

Col. Ken Christie, security chief at Direct Support Command headquarters here, said later that a Marine unit on the fringes of the camp had fired the flares that signal a gas attack. Word had been radioed to the command center and, from there, to the Nuclear, Biological and Chemical Center, which sounded the alert and began testing for chemicals in the air.

It turned out that a nervous unit on the fringes of camp had fired the flares. Master Sgt. Charles Hill, 43, of Jacksonville, N.C., in the decontamination unit, said a chemical detector may have gone off from diesel smoke or some other contaminant in the normally pure desert air, or a Marine sentry could have panicked after hearing of the attacks on Israel and Dhahran.

Panic or not, Christie, 46, of Shreveport, La., said firing the flares was the right thing to do. "I'd rather be safe than sorry," he added. "Let's not play around with this stuff and get hurt. We've got the stuff we need, and we better use it."

The alert was a useful drill in any case, he pointed out. It also was a reminder that, despite Thursday's apparently devastating air raids on Iraqi military facilities, Saddam retains the ability to strike back. Some Marines speculated that he might continue firing occasional missiles to keep U.S. ground forces off balance.

Sgt. Whitny Williams, 28, of Long Branch, N.J., said he and his men were fully garbed in their protective gear within eight minutes of the siren's blast. They had repeated the motions so many times in training that the gestures became automatic, he explained.

"I've been in the Marine Corps for nine years, so I just put it all on," Williams said. "My people didn't panic, they just put on their gear."

Two hours after the all-clear, however, someone yelled, "Gas! Gas!" and Marines yanked on their masks and ran for their protective gear a second time. The siren, the official warning, never sounded, but the entire camp was wearing gas masks before the second all-clear was passed.

"It's like a snowball effect," said Lt. Col. Jay Vesely of Chicago. "It's a good drill. But I hope it's not going to be like the boy who cried wolf too many times."