John Paul Thibodeau, 22, a Marine corporal, scrambled down from his marksman's post atop the Beirut University library as two Israeli Jeeps moved toward the American sector off Sidon Highway. His orders were clear: keep everyone out.

The Jeeps halted at a jointly manned U.S.-Lebanese Army checkpoint, where an Israeli officer said he suspected terrorists from the Palestine Liberation Army of mounting attacks from a Moslem village near the airport.

"Is it okay if we go take a look?" he asked.

"Sorry, sir, I have my orders," said Thibodeau.

Within seconds, the young Marine would become point man in an international incident involving U.S. Marines and Israelis.

The two Israeli Jeeps inched forward with eight soldiers aboard. Wary Lebanese troops looked on, their weapons "locked and loaded."

Thibodeau's .45 pistol was holstered, a loaded magazine in his pocket.

He said one Israeli "had his rifle pointed at me."

"Tell your driver to back up," he ordered.

But the Israeli officer "just smirked," he said. Thibodeau, an expert marksman from Framingham, Mass., jumped in front of his creeping Jeep.

It stopped momentarily, then bolted forward, hitting Thibodeau just below the knees and knocking him back. He didn't fall.

"Get the hell out of here!" shouted Thibodeau, whose account was confirmed by top military officials on this sprawling base as one of several U.S.-Israeli confrontations in Lebanon. "If you want trouble, you've come to the right place. But we don't want any trouble."

The Israelis wheeled around and left.

One week later, on Feb. 2, Thibodeau's commanding officer, Capt. Charles Johnson of Lima Company, 3rd Battalion, Eighth Marines, loaded his .45 and held it at the ready position to face down three Israeli tanks trying to cross a similar checkpoint.

"I knew they were just testing us, playing games to see how far we'd go," said Thibodeau, the sleeves of his fatigues rolled up to expose a taut bicep tatooed with "Mean Marine" and a helmeted bulldog. "But I was limping for two days."

The 3rd Battalion, Eighth Marines ("Three-Eight" in the verbal shorthand of the corps) is back after five months as referees half a world away, their stories of peace-keeping duty in Lebanon spilling out in the gray drab barracks by day, in the strip joints of nearby Jacksonville, where they pull liberty at night.

Few ever faced down anyone. Some say they were called "chicken" by the Israelis, who sometimes made friendly, if obscene, gestures. Marines say they returned the salutes.

But most days for the third batch of Marines to rotate back here were spent sweeping mines, showing the flag on infrequent patrols, filling sandbags and complaining about no liberty. French and Italian troops were not so restricted.

Still, life in Beirut surpassed "picking up cigarette butts at Camp Lejeune," reflected Mike Perry, 23. "At least we were earning our paycheck. A few times you got scared when there were stray rounds. But there weren't any heroes."

They awoke to gunfire in the hills, relished cheers by civilians (some of whom began sporting short, Marine-style haircuts) and hero worship by the Lebanese army. Boredom was the enemy.

"All we did was guard a road by the airport, played football, softball and frisbee," said mortarman Alan McCrary, 22. "It wasn't what you read about in books about Vietnam. It was a total letdown."

"We used to joke, 'I think I'll write someone a ticket today,' " echoed Michael Antunes, 22, a wireman for a communications unit. "You felt like a traffic cop. It got boring sitting there day after day."

"Lebanon was a test of patience," Antunes said.

Officers emphasized restraint. "We spent hours explaining the rules of engagement," said Lt. Col. John (Blackjack) Matthews, 41, a stocky two-tour Vietnam veteran who commanded the recently returned ground troops.

"We weren't going to initiate any action unless there was sufficient provocation," he said. "I kept reminding our men, 'The Israelis are not our enemy, and I don't want you to think they are our enemy.' You see them flying F16s and firing 155s howitzers made in America. We're supposed to be friends."

Yet there was always the danger of Marines getting caught in a crossfire between Israelis and unseasoned Lebanese troops "overconfident" from Marine training. Had Marines been shot, for example, when Capt. Johnson stopped three Israeli tanks, "we had the full capability of taking out the tanks," said Matthews.

There was reason for alarm. "Until we got it straightened out, the Lebanese always started clicking their weapons and locking their bolts when the Israelis showed up" at joint U.S.-Lebanese checkpoints, said one officer. "We told the Lebanese that could be seen as a provocation by the Israelis , and we weren't going to back a provocation."

Marines had orders to return fire only if being fired upon would put them in mortal danger. From his perch, Thibodeau, a crack shot who bagged Maine's "Big Buck" trophy for bringing down the state's biggest deer in 1981, commanded a view from the beaches to the hills. But he had orders to fire only if he saw a Marine shot.

"Every day, we were told not to lose our tempers," said Mark Wilson, 21. who didn't like carrying his ammunition in his pockets. "It takes two to five seconds to load, and by that time you're dead."

Yet the corporal echoed his commanders' complaints that "indiscriminate" firing by Israelis to flush out potential Palestinian ambushers endangered Marines and civilians. "If I got killed by a stray round, what reason would I have died for?"

Five Marines have been wounded by a grenade and one has been killed dismantling a mine since the Americans joined the international peace-keeping force Sept. 21. "At any minute, you felt you could be blown away," said Capt. Dennis Judge, 29, who made frequent Jeep forays to town as supply officer.

"The Lebanese were very friendly," he said. "Kids always gave me five, but you never knew if they were going to put a grenade in your hand. Traffic disputes were settled by yelling and waving guns. It was Dodge City."

Yet he navigated violence and local custom to do his job.

He swapped C-rations for sheets of plastic to cover bombed out windows, turning abandoned buildings into barracks.

But local Moslems rebuffed his offer of bottles of wine for nails. "They just shook their heads. They didn't drink."

As part of the multinational peace-keeping force their job was security. Marines dug in at the airport to keep it open, and bivouacked at Beirut University. Whenever anyone suspicious was stopped, Maj. Tom Harkins, 36, an Arab linguist who was chief translator, showed up to ask a few questions.

He was especially curious about a white Mercedes, stopped last month as it cruised up and down the beach road near the spot where a car bomb exploded two days after his troops arrived. "He said he had just gotten his car from the garage and was road-testing it," said Harkins. "But we called the garage and they'd never heard of him."

No bombs were found in the car, but he was taken away, one of 10 suspects detained by Three-Eight since they arrived last fall.

For five months, Wilson lived in a tent at one end of Beirut International Airport, nicknamed "Mortarville." One day, his company thought they were under attack until they were told it was a 21-gun salute for the Lebanese president. Another time an unidentified howitzer shell landed within 1,000 yards of his tent.

"The few times we went on full alert, we were ready to shoot. Everyone was psyched," he said. "But you were nervous the whole time. You'd go to sleep hearing rounds, wake up hearing rounds. We were ready to leave."

For social life, Marines were limited to makeshift clubs set up in bombed-out buildings or tents. Beer was sold for 50 cents, limit two per day. For music, there were tapes, or the BBC. Beirut was off-limits.

"There's no liberty, so you might as well adapt," Lt. Col. Matthews told his troops, "We're going to show the Lebanese we can be tougher" than the French and Italians.

"Most took it as a challenge once you put it that way."