A turning point in one of history's most impressive air campaigns came on the ground.

For the past two weeks, NATO military officials said, a resurgent army of ethnic Albanian guerrillas in Kosovo managed to flush out Serb-led Yugoslav troops dispersed around Mount Pastric near the Albanian border. That created the kind of "target-rich environment" that NATO pilots had been searching for during more than 31,000 sorties over Yugoslavia in the past 10 weeks.

The sudden, apparent submission today by Yugoslav President Slobodan Milosevic came in the wake of enormous losses of government tanks, artillery and ground forces in recent weeks, according to the officials. Among the 4,000 Yugoslav troops killed and 6,000 wounded during the air offensive, they said, a disproportionate share of the casualties occurred within the last few days.

"There's only one reason why Milosevic caved in to NATO's demands after rejecting them for so long," observed a senior NATO diplomat. "He realized he could not survive against the combination of attacks from the world's best aircraft and the Kosovo Liberation Army, which he personally revived through his crimes against their people."

The losses on the battlefield, along with the destruction of Yugoslav oil refineries, bridges and power stations, provided a powerful motive for Milosevic to sue for peace. Another compelling factor was a dramatic diplomatic offensive that brought together Russia and the Western powers in an unusual display of coercive clout.

With ominous foreboding, U.S. and allied officials have been warning for weeks that Milosevic's truculent rejection of NATO demands and the pressing need to return nearly 1 million ethnic Albanian refugees to their homes in Kosovo before winter could escalate the war toward a dangerous new phase -- possibly involving a ground invasion by Western forces.

Fearful that mediation efforts by his special Balkans envoy Viktor Chernomyrdin appeared on the verge of failure, Russian President Boris Yeltsin ordered Chernomyrdin, Prime Minister Sergei Stepashin and Foreign Minister Igor Ivanov to a meeting Sunday in Moscow to fashion a strategy that would satisfy NATO's terms while upholding Russia's basic demand that it end the airstrikes.

Although the ailing Yeltsin was not present at the discussions, U.S. officials described his intervention as crucial in salvaging the faltering peace mission and said it apparently was driven by a Russian need to justify its perception of itself as a great power. With the summit conference of leaders from the Group of Seven industrial democracies and Russia on his agenda later this month, Yeltsin clearly yearned to show up at the meeting in Cologne, Germany, in the role of a successful peacemaker, winning the respect of his peers.

After marathon discussions in Bonn with the European Union envoy Finnish President Martti Ahtisaari and U.S. Deputy Secretary of State Strobe Talbott that lasted right up to their departure for Belgrade Wednesday afternoon, Chernomyrdin declared that enough progress had been made in bridging their differences to present a common front -- consisting of a two-page document -- to Milosevic and other Yugoslav leaders.

He and Ahtisaari flew separately to the Yugoslav capital and immediately headed into a meeting with Milosevic. A shrewd Finnish diplomat who was elected president after a successful career at the United Nations, Ahtisaari was brought into the mediation mission because U.S. and allied governments were concerned that Chernomyrdin had not properly conveyed their demands during four previous trips to Belgrade.

Speaking tonight in Cologne, where he was briefing the 15 EU leaders upon his return from Belgrade, Ahtisaari agreed it was necessary for everybody to communicate the same message to Belgrade. "To me, it was absolutely vital that there be no doubt and no misunderstanding about what was being discussed. All of us needed to be in the same room," Ahtisaari said.

When he presented the peace package in Belgrade Wednesday evening, in a meeting that lasted from 5 p.m. to 9:30 p.m. (11 a.m. to 3:30 p.m. EDT), Ahtisaari made a point of reading the entire document aloud.

The Finnish leader said there was never any question of negotiating with Milosevic. "My role was to answer questions about the document, and there were plenty of them," he said. The most pointed was whether he could improve on the conditions that the paper laid out.

"I had to say it was the best offer the international community was in a position to make," Ahtisaari said. Rather than say more, he urged Milosevic to discuss the terms with members of his government and deliver his decision at a meeting the following day.

When they gathered this morning, Ahtisaari was peppered with more questions that he characterized as "very matter of fact." The Yugoslavs wanted to know the precise role of the United Nations in the international security force that would enter Kosovo and whether the earlier peace accord worked out at Rambouillet, France, but not signed by Belgrade, still had any validity. Ahtisaari did not tell reporters what his answers were.

Two subjects were not raised by either side: Milosevic's indictment on war-crimes charges last week by an international tribunal in The Hague, and whether the Yugoslav president still held the right to remain in office. After the Serbian parliament gave its approval, Ahtisaari went back to Milosevic's office shortly after 1 p.m. (7 a.m. EDT) and walked out with his agreement a half-hour later.

In increasing military pressure on Belgrade, NATO officials emphasized what they described as their recent battlefield successes working with the Kosovo Albanian guerrilla army despite the alliance's official arm's-length relationship with the rebels. NATO commanders had complained that the alliance's effort to impose its will through air power alone made their jobs difficult; there were no NATO ground forces to flush Yugoslav soldiers, tanks and artillery out of hiding in Kosovo or force them into larger formations so alliance warplanes could hit them.

In recent days, however, the guerrillas launched a ground offensive from Albania into Kosovo, threatening to divide Yugoslav forces by capturing a strategic road between the towns of Prizren, in southern Kosovo, and Pec, in the west.

Yugoslav forces were able to repulse the guerrilla offensive, but to do so, a NATO military official said, they massed together "like in a hog pen." They then became relatively easy targets for airstrikes, especially by A-10 "Warthog" tank-killing planes, he said.

The news that Milosevic had finally buckled received a cautious welcome today at alliance headquarters here in Brussels, reflecting an abiding distrust toward Milosevic's intentions and deep anxiety about the difficulties facing the 50,000 peacekeeping troops destined to be sent into Kosovo.

Ahtisaari said he hopes that NATO military commanders will meet as soon as possible with their Yugoslav counterparts to resolve the sequence and timing of the government troop withdrawal and a suspension of NATO airstrikes. NATO spokesman Jamie Shea said the airstrikes will not be called off until a withdrawal from Kosovo by the 40,000 Yugoslav troops and Serbian special police is well under way. "For now, operations will continue," Shea said.

NATO military commanders say they may face problems in getting Yugoslav and Serbian forces to clear their own land mines as stipulated under the accord and in persuading Kosovo Liberation Army members to surrender their weapons just as they feel they are regaining momentum in their fight to establish an independent homeland.

The United States and its allies have insisted all along that Kosovo should remain part of the republic of Serbia, which along with Montenegro forms the Yugoslav federation. But after all the suffering endured by the Kosovo Albanians, NATO military officials acknowledge that even under the temporary protectorate to be secured by allied soldiers, it will be difficult to persuade the guerrillas and the growing number of refugees who support them to abandon their goal of independence.

"When we look back on this conflict, the air war may be considered the easy part," said a senior NATO military officer. "It is going to be much harder to get these people to forget the violence and live in peace."

Drozdiak reported from Brussels, Swardson from Cologne. Correspondent David Hoffman in Moscow and staff writer Thomas W. Lippman in Washington contributed to this report.

CAPTION: Residents of the Yugoslav town of Velika Plana look at a viaduct damaged in overnight NATO bombing. Attacks are set to end under yesterday's accord.

CAPTION: European Union envoy Ahtisaari talks to reporters after meeting with Yugoslav President Milosevic.