NATO Takes Command of Bosnia Peacekeeping : Military: British and American forces plunge into Serb-held areas. Troubled U.N. mission ends its role.

Embarking on the largest operation of its history, NATO took charge of enforcing peace throughout war-weary Bosnia-Herzegovina on Wednesday in a mission that will send 20,000 American troops into action in Europe.

Within hours of the NATO takeover, British and U.S. forces breached quiet battle lines and plunged into Serb-held territory, and roadblocks that came to symbolize the war’s cruel restrictions on movement were being dismantled.

At a brief military ceremony delayed by bad weather in Sarajevo, the United Nations’ troubled 3 1/2-year role in the conflict came to an end as U.S. Navy Adm. Leighton W. Smith assumed authority over the men and women who will keep apart Bosnian enemies and help rebuild a destroyed country.

“This event . . . marks a historic occasion,” Smith said of what he called the North Atlantic Treaty Organization’s “defining moment.” “It represents opportunity, an opportunity to move from the ravages of war to the benefits of peace.”

NATO is charged with executing a comprehensive peace treaty aimed at ending Europe’s bloodiest conflict since World War II, a savage neighbor-against-neighbor war that killed as many as 250,000 people and drove up to 3 million others from their homes. The treaty was signed by the presidents of Bosnia, Serbia and Croatia last week in Paris.

Keen to assert their authority and dramatize the changing of the guard, British troops based in central Bosnia on Wednesday became the first NATO contingent to post itself in Serb-controlled territory, venturing into land the United Nations generally had to avoid.

Two British motorized patrols were operating about 20 miles south of the Serb stronghold of Banja Luka, in north-central Bosnia, and were greeted by smiles from civilians, British army spokesman Maj. Lindsay Rumgay said.

Meanwhile, a convoy of U.S. GIs in Humvees, led by the senior American commander in the field, Gen. William Nash, forged across the volatile Serb-held Posavina corridor to hook up with troops preparing to build a pontoon bridge across the Sava River.

Earlier, John Menzies, the U.S. ambassador to Bosnia, made what is believed to be his first official visit to Serb territory, paying calls on the mayors of Ilidza and Grbavica, two Serb-held suburbs of Sarajevo. The trip was made Tuesday but was not disclosed until Wednesday.

“The idea,” Rumgay said, “is to establish a presence.”

In Sarajevo, jaded residents watched with interest as NATO’s camouflage-green jeeps zipped down snow-coated streets.

“We welcomed the U.N. with flowers, but they proved themselves to be a disaster, and we were disappointed,” said Atif Delalic, a retired economist living in a bleak apartment complex routinely pummeled by Serb tank fire. “But with NATO, it’s a new moment for Bosnia. We get an impression of power.”

The war left this once graceful, cosmopolitan capital, as well as scores of other Bosnian cities and villages, in ruins. Another casualty was the U.N. peacekeeping mission, which found itself repeatedly humiliated and ultimately impotent.

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Reflecting the moral anguish of the failed U.N. mission, Kofi Annan, head of the U.N.’s peacekeeping division and special envoy for the former Yugoslav federation, urged a period of soul-searching to prevent another debacle like Bosnia.

“The world cannot claim ignorance to what those living here have endured,” he said at the ceremony marking the formal transfer from the United Nations to NATO.

“In looking back we shall all record how we responded to the escalating horrors of the last four years. And as we do so, there are questions that each of us will have to answer. What did I do? Could I have done more? And could it have made a difference?

“Did I let my prejudice, my indifference and my fear overwhelm my reason? And how would I react next time?”

NATO officials believe that they have a better chance at success than the United Nations ever had, for several reasons.

An authentic peace treaty exists that has been signed by all parties--although the Bosnian Serbs were represented by the president of neighboring Serbia, Slobodan Milosevic, a designation of powers that rebel leaders now regret.

NATO will have three times the number of troops that made up the U.N. force; by contrast, it is heavily armed and has a clear, more efficient command structure.

But on his first day as commander of the NATO operation, Smith immediately got a taste of the kind of uncontrollable menaces that plagued his predecessors: The ceremony marking the transfer of authority had to be delayed for four hours when the admiral’s plane could not land in Sarajevo because of soupy, freezing fog.

He circled for about an hour before diverting to the coastal Croatian city of Split, and then he was helicoptered to Sarajevo. Among the departing U.N. officers, there was a sense of small vindication.

The ceremony itself was a sparse memento of war. It was held in the gutted, bombed terminal building at Sarajevo’s ravaged, sandbagged airport. Water dripped from leaky pipes, and guests kept their winter jackets on against the draft. Colorful parachutes were draped above and behind the dais to give a semblance of decoration.

For the most part, the arrival of NATO meant that the British, French and other European peacekeepers who were part of the U.N. force simply exchanged their blue helmets for camouflage ones.

White U.N. vehicles were being painted green, U.N. insignias were scraped from armored personnel carriers and replaced with “IFOR” (Implementation Force, the official name of the NATO operation), sometimes written with black masking tape.

In Tuzla, the northeastern city that will be headquarters to the American contingent, the U.N. sign at the Tuzla air base was removed and replaced with one bearing the IFOR eagle.

NATO is calling its operation Joint Endeavor. Officers wear armbands with the IFOR acronym in both the Roman alphabet and the Cyrillic alphabet used by the Serbs.

With Wednesday’s hand-over, attention now shifts to the tasks that lie ahead.

Among numerous timetables laid out in the peace accord, a seven-day deadline was set for the warring parties to evacuate selected areas around Sarajevo. Under the agreement, Serb-held suburbs around Sarajevo are to revert to government control within 90 days, and many Serbs are resisting the prospect of being ruled by an enemy.

Smith acknowledged that the capital poses one of the most delicate problems in the peace accord, and, in a news conference, he hinted that some deadlines will be extended.

Other deadlines that loom include a 30-day period for all armies to withdraw beyond designated zones of separation and for all foreign armies and paramilitary forces to leave the country. Exchange of all prisoners is also to take place within 30 days.

At least as daunting as the military mission is the civilian side of the treaty, which calls for refugees to return home and elections to be held within nine months.

Smith emphasized that IFOR will apply “evenhanded” treatment toward all sides. While his troops are prepared to use force to defend themselves, Smith said he wants to persuade Bosnians that NATO is here to help. “We’re not here as a bunch of cowboys looking for a fight,” he said.

Times staff writers Tyler Marshall in Zagreb, Croatia, and Richard C. Paddock in Tuzla contributed to this report.