As show-of-force air exercises move NATO closer to intervening against Serbia's onslaught in Kosovo province, the US and its allies confront considerable political and military risks.
Unlike in Bosnia-Herzegovina, NATO intervention in Kosovo would be undertaken without a peace accord, making enormously uncertain its costs, duration, and prospects for success.
NATO action in Kosovo also raises important international political issues, including relations with Russia and the United Nations, and the alliance's role in post-cold-war Europe in the absence of the Soviet threat it was formed to repel.
Those unknowns and other concerns make some American officials deeply uneasy.
"I'm very skeptical of using force to intervene in a way that does not support a clear objective on the ground," says one senior US official. "In Bosnia, we used power to shore up a state and ensure its integrity. What would we be doing in Kosovo?"
But the Clinton administration, after months of often confused diplomacy, has been forced to face the danger of the conflict exploding into neighboring Albania and Macedonia, igniting a regional war that could suck in NATO members Greece and Turkey.
In an escalation of pressure on Yugoslav President Slobodan Milosevic to call off an offensive in Kosovo and hold peace talks with ethnic Albanian leaders, 40 aircraft from the United States and other NATO states were to conduct simulated airstrikes today over Albania and Macedonia.
The exercises were ordered during a NATO meeting last week to back up a threat of intervention. The alliance's defense ministers also directed military planners to work up options for airstrikes and ground operations.
Their decisions were endorsed Friday by all the members of the Contact Group on the former Yugoslavia - the US, Britain, France, Germany, and Italy - with the exception of Russia.
Though no deadline was set for Mr. Milosevic's compliance, it appears the United States and its allies will wait for the conclusion of talks he is to hold with Russian President Boris Yeltsin in Moscow this week.
Critics of intervention cite serious obstacles, including the lack of a peace accord between Milosevic and rebels of the Kosovo Liberation Army. The KLA's fight for independence from a decade of harsh Serbian rule is supported by most of Kosovo's 2 million ethnic Albanians, who outnumber Serbs there 9 to 1.
Intervention would almost certainly mean clashes with the 140,000-strong Yugoslav military. Though poorly funded and dependent on large numbers of conscripts, it has capable air defenses, more than 200 modern M-84 tanks, and a handful of Russian-made MiG-29 jet fighters.
NATO attacks on Yugoslav forces might also help the KLA's fight for independence, an outcome opposed by the US and its allies. They favor restoring to Kosovo the autonomy Milosevic revoked in 1989, but keeping it part of Serbia, which with Montenegro makes up what is left of Yugoslavia.
Should they intervene with the objective of promoting that solution, NATO forces initially welcomed as liberators by ethnic Albanians could find themselves under attack as an occupation force, some experts warn.
Despite the unusual display of unity, differences persist within NATO over the question of whether it can intervene, without UN Security Council approval, in a sovereign state outside the alliance's boundaries to ensure regional stability.
FRANCE and Germany, as well as nonmember Russia, insist on a United Nations resolution.
"We should not create a precedent when NATO acts outside the territories of its member-states without a relevant decision by the UN Security Council," insists Russian Foreign Minister Yevgeny Primakov.
But the US and Britain say that while a resolution is desirable, it is not necessary as the UN Charter allows regional groups to intervene in internal conflicts that threaten regional stability.
"To subordinate NATO's concern for security ... to the United Nations is inadvisable and not necessary," contends US Defense Secretary William Cohen.
Finally, a NATO mission in Kosovo would further strain relations between the US-led alliance and Russia. Moscow considers the Balkans its traditional bailiwick and is still incensed over the alliance's decision to expand into its former Eastern European empire.
Supporters of NATO intervention acknowledge the risks and uncertainties. But they say that only NATO can halt the slaughter and expulsions of ethnic Albanian civilians and restore the calm required for meaningful peace negotiations.
"The objectives of NATO involvement would be quite limited: a cease-fire, the removal from the theater of the forces creating the carnage, and a commitment by the parties to a serious dialogue with international mediation," argues Ivo Daalder, a former National Security Council staffer.
Since they launched their offensive some two weeks ago, Serbian forces have attacked civilian centers along Kosovo's border with Albania in what the US decries as "ethnic cleansing." Hundreds of people are believed to be dead, while an estimated 65,000 others have been driven from their homes, many forced to go to Albania.
Intervention advocates argue that the costs to the US and its allies of acting now would be much lower than those of having to intervene to halt a regional war.