WASHINGTON -- With Bosnian peace talks in a crucial phase, a key weakness in U.S. strategy is becoming clear: Serbs will face the introduction of U.S. and NATO guns only if they join in an agreement; if they continue to balk, they won't.
If all three warring factions, including Serbs, sign on to the remaining elements of an international peace plan, the Serbs will face strong military pressure to give up their heavy weapons and territory gained in their yearlong war against Bosnia's Muslims and Croats.
The United States and its allies plan to dispatch a muscular force to implement the deal, heavily armed, numbering in the tens of thousands, given enough leeway under rules of engagement to suppress violations of an agreement, diplomats say.
"I'd be pretty dubious that they have any strong incentive or that there'd be any consensus" among Bosnian Serbs to endorse the peace plan, said Jack Seymour of the Atlantic Council, who directs a program on U.S.-European relations.
In place of military force to get the Serbs to sign, Western diplomats see further isolation of Belgrade and of the government led by Slobodan Milosevic as the key to bringing his allied Bosnian Serbs into line.
It would be, says special U.S. envoy Reginald Bartholemew, "a formal diplomatic isolation that the world hasn't seen."
Mr. Milosevic is to meet in Paris tomorrow with peace mediators Cyrus R. Vance and Lord Owen. Mr. Vance said the meeting is designed to pressure the Serbian president to influence Bosnian Serbs to go along with the mediators' peace plan.
In addition to tightening already existing U.N. sanctions on Serbia, the United States and its allies are looking for ways to block Serbian financial transactions and money sent home by Serbians abroad. They also contemplate a virtual sealing of Serbia to all transit, communications and postal service.
The question of when to apply force assumes new urgency now as peace talks mediated by Mr. Vance and Lord Owen at the United Nations enter a crucial stage. Bosnian Muslims have accepted all but one portion of Vance-Owen peace plan and appear inclined to approve a 10-zone map. This would leave Bosnian Serbs as the only holdouts.
If the peace talks collapse now, U.N. diplomats fear, Bosnian carnage is likely to mount in spring offensives.
The peace-enforcement troops contemplated, including contributions from the United States, other NATO countries and Russia, would confront the Serbs with a potent force.
But President Clinton and President Francois Mitterrand of France, following a morning meeting at the White House yesterday, reiterated the West's refusal to enforce terms of the Vance-Owen plan until all parties agreed.
Mr. Clinton told reporters they had agreed "that it would be an error for France to increase its troops or for the United States to introduce troops to become embroiled in the conflict, but that we both should be prepared to make our contributions to enforcing an agreement . . . if the Vance-Owen process could produce one."
Critics of U.S. policy point to this refusal as an "escape clause" that allows the Serbs to stall any introduction of U.S. forces while continuing their gains on formerly Muslim-controlled territory.
As peace talks broke up in New York last weekend, Bosnia's Muslim president, Alija Izetbegovic, gave the strong impression that he intended to return in a matter of days after consulting with colleagues and sign the accord.
Diplomats close to the talks temper their optimism, saying there are many in the Muslim camp holding out for a military solution.
But they see pressure building on the Bosnian Muslims from a softening of some Muslim countries' opposition to the Vance-Owen plan and signs that the Muslim-Croat military alliance may not hold up indefinitely.
Although the Vance-Owen plan has been criticized for ratifying some territory gained by the Serbs through ethnic cleansing, it would leave them in control of only 43 percent of the country in comparison with the more than 70 percent they hold now.
In addition, it would cede to Muslims, who traditionally have lived in cities and large towns, much of the economically more productive industrial zones.
Diplomats hope that Serbs can be persuaded in part by the human-rights underpinnings of the plan that guarantee minority rights in all 10 zones.
If the Muslims sign the agreement and the Serbs don't, U.S. and European officials are considering seeking U.N. Security Council endorsement of the Vance-Owen plan to increase the diplomatic pressure.
A U.S. official said British and French opposition to enforcing a no-fly zone over Bosnia or even arming Bosnian Muslims might diminish if Serbs are the only holdout.
But at the same time, with the help of Russia -- a traditional friend to Serbia -- they have to offer an incentive to Belgrade: the prospect of being readmitted into the international community.