NATO's deepening involvement in the Balkans is having a profound effect on the alliance's future, diverting its attention and resources to southern Europe and increasing the chances that nations there will be among the next to join the pact, according to senior NATO diplomats and military experts.

After training for decades to defend against a Soviet-led invasion and other threats in northern and central Europe, the alliance is now dramatically shifting its military assets. More than 30,000 allied troops are now based in Bosnia, and 29,000 others--more than half of an eventual 57,000-member peacekeeping force--have reached Kosovo. In addition, 10,000 troops are deployed in Macedonia and another 7,500 in Albania.

Beyond the multitudes of troops and vast sums of money that will be required to secure peace in Kosovo and Bosnia for years to come, NATO officials say the alliance will soon begin debating whether the best way to ensure lasting peace in southeastern Europe might be to make several of Yugoslavia's neighbors full-time members.

While NATO--which grew to 19 members this year with the inclusion of Poland, Hungary and the Czech Republic--has agreed not to consider additional expansion until the year 2002, the Kosovo conflict has bolstered the stature and strategic importance of Slovenia, Romania and Bulgaria.

At the same time, NATO's strained relationship with Moscow has contributed to hampering membership prospects for the Baltic states--the onetime Soviet republics of Latvia, Lithuania and Estonia. They border Russia, which would be irritated if they joined. The shift in NATO thinking comes despite the relative success of the three nations in their transition to free-market economies and in easing ethnic tensions with Russian minorities since regaining independence in 1991.

"We all realize that the sources of instability are in the south, not in the north," said a senior NATO official. "The question we need to consider is whether the alliance should embrace those who are weak and in need of help or accept those who have few problems and would be easy to digest."

NATO Secretary General Javier Solana, who plans to visit Romania and Bulgaria this week to express his gratitude for their assistance during NATO's 78-day air war against Yugoslavia, insists the alliance must recognize the depth of its commitments in the Balkans and the prospect that southeastern Europe will remain the alliance's top priority for the foreseeable future. Solana and other advocates of a "southern strategy" point to NATO's success in thwarting open conflict on several occasions between Greece and Turkey, historic rivals who joined NATO in 1952.

The rising prospects of the southern European countries have dismayed leaders of the Baltic states, who believe they are being pushed unfairly to the back of the line. Latvia's outgoing president, Guntis Ulmanis, whose country agreed to ease citizenship requirements last October for more than 600,000 stateless Russian-speakers, said the Baltic nations deserve greater credit that they have been given by the West for the way they have dealt with their own crises despite being left outside NATO and the European Union.

"We can note with considerable pride how the Baltics can serve as a model for Europe on how to secure a stable, peaceful and prosperous life, along with tolerance among minorities that is so lacking in the Balkans," Ulmanis said at a farewell news conference last week.

The three Baltic states are active participants in the Partnership for Peace program that NATO established to increase military cooperation with its former enemies in the Warsaw Pact, now defunct. But the alliance has created a new forum with seven Balkan countries--Slovenia, Bulgaria, Romania, Macedonia, Albania, Croatia and Bosnia--that NATO officials say could emerge as a key test of the alliance's commitment to the region.

The Balkan Stability Pact, which was unveiled at NATO's 50th anniversary summit meeting in Washington in April while the air war against Yugoslavia was in full swing, pledged a new level of economic and military assistance to Belgrade's immediate neighbors. But now that the war is over, whether NATO makes good on those promises remains open to question at a time of competing pressures on alliance military budgets.

With Solana expected to leave NATO in October for his new post as foreign policy czar of the European Union, the fate of the alliance's "southern strategy" could hinge on the choice of his successor. While no clear favorite has emerged, leading candidates include Denmark's savvy defense minister, Hans Haekkerup, and the former leader of Britain's Liberal Democratic Party, Paddy Ashdown.

Haekkerup has been a prominent supporter of a Baltic security zone and is known to be a close friend of U.S. Defense Secretary William S. Cohen, whose support could be crucial. Ashdown, on the other hand, lacks government experience but has been a stout advocate of the view that NATO must show vigorous leadership in extirpating Yugoslav President Slobodan Milosevic and bringing an enduring peace to the Balkans.

Another possible candidate is Polish Foreign Minister Bronislaw Geremek, a former leader of the Solidarity trade union movement. Despite a lack of experience in alliance matters, Geremek was cited by several NATO diplomats as a charismatic figure whose selection could send a positive signal to all central and East European countries hoping to become NATO members.

Geremek has been an active supporter of NATO involvement in the Balkans and maintains close ties to his Baltic neighbors. But NATO diplomats concede that the idea of a Pole heading NATO's political administration might be difficult for the Russians to accept--almost as hard, they say, as consenting to inclusion of the Baltic states in the alliance.

CAPTION: French Gen. Bruno Cuche, left, escorts NATO Gen. Wesley Clark around Kosovska Mitrovica, a Kosovo city divided between Serbs and ethnic Albanians.