When Serb fighters shelled an isolated U.N. observation post near here last Friday, it was an unremarkable event. In nearly two years of U.N. operations in Bosnia, this country's combatants have routinely shelled, sniped and humiliated U.N. soldiers -- even sometimes robbing them at gunpoint of their weapons and uniforms. But Friday night, the Serb fusillade elicited a U.N. response more incendiary than the usual verbal protest. A cigar-chomping, karate-kicking Danish officer, Lt. Col. Lars Moller, ordered his white-painted Leopard tanks to fire back at the Serb artillery position. By dawn, the Danish army had fought its fiercest battle since the Nazi invasion of 1943 -- and Moller had shown that the United Nations need not always retreat in the face of frequent military challenges by the Serbs. Moller, who speaks fluent English with the accent of Sean Connery and the slang of a U.S. Marine, said his tour in the Balkans has taught him that "if you are scared down here, you're going to get kicked. That's the way it works." In Bosnia's civil war, he said, "all sides are full of a lot of macho bull. . . . You have to adjust your behavior accordingly."

The Danes' destruction of a Serb artillery position underscored the dilemma of U.N. peacekeeping. U.N. soldiers and foreign aid workers here often express frustration with the U.N. forces' passivity in the face of provocations. But the Danes' momentary aggressiveness was similar to that attempted as policy -- and abandoned in frustration -- by last year's peacekeeping mission in Somalia. Moller, for example, does not view his troops' riposte, in the end, as a victory. Despite his own version of macho bluster, he sees it as a defeat. By finally shooting back, Moller and his troops destroyed bridges of confidence and trust that they painstakingly had built to the Serb side. The Danes had built a four-mile road for Serb children to use in walking to school from the nearby village of Pelemsi, so they would remain safe from Muslim shelling. They had arranged shipments of diesel fuel to Serb road repair crews and seeds to Serb farmers. But, Moller said, given the bellicose psyche of this region, its deadly macho games of chicken and its adoles cent tests of strength, he cannot regret giving the order to fire. "The U.N. should not bow its head to any of these people," he said. "Once you do that, you lose your dignity and, even worse, the other guy will keep walking over you. In the Balkans, you've gotta stand tall." Standing tall has been difficult for the Nordic Battalion -- a U.N. unit composed of 1,753 Swedes, Danes, Norwegians and Dutch -- that is based around the Muslim-held region of Tuzla. Over the last two months, Serb gunfire has destroyed four of the battalion's armored personnel carriers. By luck -- an open safety hatch here, a poorly aimed rocket there -- no one has been killed. Although the Serb attacks have grown intense, the U.N. political command in Zagreb, Croatia -- directed by special envoy Yasushi Akashi -- has rejected at least four of the battalion's requests for NATO planes to fly close air support for U.N. troops here, Moller said. In one incident, on March 18, Serb fighters destroyed a Swedish armored personnel carrier with an antitank missile and six tank rounds in the northern town of Gradacac. Moller reported it, clearly blaming the Serbs -- but U.N. officials in Zagreb contended that the source of the attack was unknown. On April 14, Serb artillery shelled Tuzla's airport for four hours, in violation of a U.N. Security Council resolution ordering the Serbs to allow the airport to open or risk NATO airstrikes. Since the shelling, the airport has remained closed. But on Friday, the United Nations' response came not from Zagreb or New York. It came from the Danish soldiers in their 43-ton, German-built Leopard tanks, the most advanced weapons system of the meager U.N. arsenal in Bosnia. Since the Leopards arrived in the Balkans in the middle of last year, they have been a target of the Serbs. For four months, as the United Nations tried to move the tanks here through Serbia, the government there held them up outside Belgrade. Finally, the United Nations sent them to Split, Croatia, and drove them to Tuzla in March. Since then, the Serb forces that ring the Tuzla region have declared open season on the Leopards. Danish army Maj. Carsten Rasmussen, who commands the tank squadrons, said Serbs had fired on the tanks a dozen times. Once, he said, in early April, the Danes fired back destroying a Serb bunker and a 40mm antitank gun. It was only a matter of time, Rasmussen said, before a major clash would erupt. Shortly after 11 p.m. Friday, Serb gunners around Mount Vis, to Tuzla's south, opened up on a U.N. observation post called Tango 2. Since October, according to U.N. figures, the Serbs had shelled the post 28 times with 96 shells. As they always do, the Leopards responded. Moller and his men sped east from Tuzla in seven tanks and two armored personnel carriers. At the village of Saraci, in view of the Serb gunners, the Danes stopped and -- in accord with U.N. rules of engagement -- illuminated their white vehicles with searchlights to let the Serbs know they were there. The lights drew shellfire. One Serb shell landed 30 feet from Moller's vehicle, he said. Others blew metal shrapnel over the tanks. "At that point we turned the lights off," Moller said. "Goooood thinking, as the Brits say." With that, Moller's Operation Hooligan Buster began in earnest. The Danes had practiced the routine. Four of the Danish tanks and an armored personnel carrier sped to another village, Kalesija, which was closer to Tango 2. The Serbs responded by lighting up the night with artillery and rocket fire. An antitank rocket erupted 15 feet behind one tank, Moller recalled. Arriving in Kalesija, Rasmussen moved two tanks up the hill toward the beleaguered observation post, and placed two others behind houses in the village. Then, the Danes said, the Serbs stepped up the attack, firing 40mm antitank cannons. By then the Serbs had been firing for 30 minutes, the Danes said. When the troops in Saraci reported that more antitank rockets were on their way, Moller and Rasmussen ordered the three tanks in Saraci to fire warning shots -- four in all. When the Serbs continued the attack, the Danes fired in earnest. The first round silenced an antitank gun, the second destroyed the post of a forward artillery observer -- and the third plowed through a Serb bunker, the Danes said. "Things were getting out of hand," Moller said. He and Rasmussen agreed to hold their fire and ensure that Serb shelling of Tango 2 had ceased. After 30 minutes of quiet, the forward tanks began moving back to Saraci -- but the Serbs began attacking them again. Moller said he "began to get (ticked) off." The officers ordered the tanks in Saraci to reopen fire. Continuously. For 15 minutes. One round plowed into a Serb ammunition dump, igniting a massive, concussive blast. Not since a 1943 battle against the Nazis -- and before that an 1864 clash with invading Prussians -- had Danish forces been in such a fight, the Danes reckoned. The two earlier ones, they had lost. Moller said the Danes spared three Serb T-55 tanks because, while the Leopards' infrared detectors found the Serbs' aiming systems turned on, they also determined that the enemy tanks' barrels were cold. Under the restrictive U.N. rules of engagement, only guns actually caught in act of firing may be hit. Rasmussen and Moller said Friday's ordeal was meant to be a trap for the Danes. "It was an ambush," Moller said. "Tango 2 was the cheese and we were the mouse. Only it turned out that the mouse ate the cat."

PHOTO, IN TUZLA, DANISH LT. COL. LARS MOLLER CHOMPS A CUBAN CIGAR IN FRONT OF ONE OF HIS LEOPARD TANKS. JOHN POMFRET