NEWS ANALYSIS : Shelled Market Not Sole Spur to Intervention : Bosnia: Forces have eroded the West’s largely passive position and pushed it closer to a new, more active role.

For months, as snipers picked off pedestrians and artillerymen cut down children at play in Sarajevo, the United States and its European allies seemed paralyzed--their vaunted power unused, their diplomacy wasted in empty threats and sterile bickering over whose fault it was.

Now, finally, the great powers of the West say they are getting serious about lifting the siege of Sarajevo. They are preparing a demand for the removal of artillery from the mountains around the city, and threatening air strikes to enforce the order.

What has brought Washington and its allies to the brink of action after blinking away so much violence, brutality and death? Was it really just the happenstance of a single mortar shell that fell into a crowded outdoor market last Saturday, killing 68 shoppers near the old stone bridge where World War I began?

“Don’t underestimate the impact of that,” a White House official said. “We all saw it. It killed more people than had been killed in any one incident. And it came exactly three weeks after the President of the United States told NATO that it shouldn’t threaten air strikes if it didn’t mean it.”

Yet much as the grisly scene served as a catalyst, other forces were already at work eroding the West’s largely passive position and pushing it closer to a new, more active role. The tragedy of Bosnia-Herzegovina has refused to yield to rhetoric or simple diplomacy. And it has relentlessly exposed the inability of the West’s humanitarian efforts to achieve anything like humane results.

At a tactical level, there has been a growing recognition that Sarajevo could not hold out forever. Added to that is the slow deterioration of the U.N. forces’ capacity to deliver relief aid, accompanied by escalating attacks on peacekeeping troops by all three Bosnian factions. And there was the sinking realization, as one U.S. diplomat put it, that “whatever we did, we couldn’t do much worse.”

Whether the North Atlantic Treaty Organization will actually carry out its new threat to take military action after failing to back up earlier warnings, and whether air strikes can succeed in deterring further attacks on the civilians of Sarajevo, remain to be seen.

Even less certain is whether relieving the city’s punishing siege will do much to end the war that has killed more than 200,000 people in less than two years. Some diplomats even worry that saving Sarajevo could have an unintended side effect: encouraging the Muslim-led Bosnian government to harden its stand in peace negotiations and further slow the snail’s-pace progress toward compromise.

Still, as NATO meets today in Brussels to consider a new strategy, the alliance’s key members have already moved several steps closer to military intervention against the Bosnian Serbs--an act that would be a watershed in the sad history of halfhearted Western attempts to stop the killing.

“We have moved, the British have moved, the French have moved,” a senior State Department official said. “We still hope air strikes won’t be necessary. But if you took a poll, four out of five officials working on this think we will end up using force.”

The proposed NATO plan, a combination of U.S., British and French ideas, would order the Bosnian Serbs to move all artillery and other heavy weapons at least 20 kilometers from Sarajevo--about 13 miles, a distance intended to put the city out of range. They would be given seven to 10 days to comply, officials said.

At the same time, NATO would order the Bosnian government to turn over to U.N. peacekeepers all heavy weapons inside the city.

NATO would threaten to destroy any armored vehicle, mortar or artillery piece found inside the 20-kilometer zone after the deadline, the officials said. “We have been ready for this kind of use of force for some time, but this time we think the Europeans are ready too,” a U.S. official said.

Along with the ultimatum in Sarajevo, the Clinton Administration has agreed to European pleas to work more actively for a negotiated settlement in the war, officials said. Until now, the United States attended the negotiations as an observer, but refused to give any real support to the Europeans’ peace plan, which would divide Bosnia into Serbian, Croatian and Muslim republics--accepting, in the process, Serbian forces’ conquest of large segments of what was formerly Muslim land.

Even before Saturday’s attack, the United States and its allies had been moving toward a more energetic effort to relieve the siege of Sarajevo and restart the peace talks, officials said. That work began in earnest Feb. 1, when British Foreign Secretary Douglas Hurd told Secretary of State Warren Christopher that Europe wanted more U.S. leadership on the issue, U.S. and British officials said.

Hurd also convinced Christopher that the European Union was willing to agree to a U.S. demand that the peace talks avoid putting pressure on the Bosnian Muslims.

But the weekend massacre gave both sides a mighty push, officials said. It stepped up pressure from Congress, European parliaments and the public for action. A CNN/USA Today poll released Tuesday found that 48% of Americans favored allied air strikes in Bosnia, against 43% opposed--a switch from before the massacre, when a Los Angeles Times poll found 33% in favor and 48% opposed.

U.S. diplomats were still seeking agreement from two countries reluctant to sign on: Russia, a U.N. Security Council member with historic and cultural ties to Serbs, and Canada, a NATO member that fears its 2,000 peacekeeping troops in Bosnia might become targets for retaliation if air strikes occur.

Many in Congress remained skeptical too. “If you embark on this policy, you had better be able to tell the American people what you do if it fails--what is the other option if this doesn’t work,” said Sen. John McCain (R-Ariz.), a conservative member of the Senate Armed Services Committee.

But Gen. John Shalikashvili, chairman of the Joint Chiefs of Staff, said that air strikes could be counted as a success even if they merely reduced the shelling.

“Air power alone cannot bomb someone into a peace agreement,” Shalikashvili told the Armed Services Committee.

Times staff writers Stanley Meisler at the United Nations and Art Pine in Washington contributed to this report.