NEWS ANALYSIS : Bosnia Viewed as Most Difficult Peace Operation : Pacification: Complex agreement, terrain, bitterness among foes all add to problems for international forces.

WASHINGTON —  The mission awaiting a large U.S.-backed peacekeeping force in Bosnia if a peace agreement is eventually reached there would be the riskiest and most difficult of its kind ever undertaken by the international community.

As military analysts have warned, U.S. and U.N. peacekeepers would face tortuous terrain, far more daunting than in Somalia or the Persian Gulf, that could expose them to attack and seriously impede their mobility.

And the deep-seated bitterness among the three major warring factions--the Serbs, the Muslims and the Croats--would greatly increase the chance of renewed fighting, this time with allied troops in the middle of it.

But experts cautioned that the biggest rub is in the peace plan worked out by U.N. negotiator Cyrus R. Vance and European Community negotiator Lord Owen. The accord, they say, is so complex and vaguely worded that it virtually defies serious enforcement unless all parties are on unprecedentedly good behavior.

Although Bosnian Serb leader Radovan Karadzic signed the agreement Sunday, the self-styled Bosnian Serb parliament refused to ratify the plan.

President Clinton had warned that if the Bosnian Serbs rejected the accord, the United States would consider launching air strikes at their artillery emplacements. But it still was uncertain whether U.S. allies would go along with such a move.

If the peace plan is adopted, the allies would still have difficulty achieving success by any traditional definition--that is, installing a stable, functioning government.

But in backing the peace plan as they have, the Clinton Administration and its allies have shown that they are willing to settle for simply trying to halt the Serbian campaign of “ethnic cleansing,” at least buying more time.

The peace plan hammered out by Vance and Owen sought to stop the current fighting, separate the three warring factions and divide Bosnia-Herzegovina into 10 autonomous provinces, with Serbs dominating three, Croats three and Muslims three and the capital district, Sarajevo, under control of all three communities.

According to the U.N. agreement, if the allies took on the peacekeeping job, they would have to secure the entire region militarily, remove all heavy weapons, patrol the provinces, roads and international borders and protect the various ethnic communities.

Pentagon planners said that the task would require 65,000 troops, more than double the number sent to Somalia, along with enough heavy equipment and weapons to make previous peacekeeping operations look like practice exercises.

Those force levels, however, assume that the operation will proceed with minimum disruption.

Lt. Gen. Bernard Trainor, a retired Marine Corps strategist now at Harvard University’s Kennedy School of Government, warned that, if the allies encounter serious resistance, it could require up to 130,000 troops.

“Potentially, it’s a back door for the allies getting involved in the war,” Trainor complained.

Barry Posen, a Massachusetts Institute of Technology military expert, argued that, even from a purely political viewpoint, the peace plan is so unrealistic that it would virtually invite violations by the Serbs.

The accord would require Serbian nationalists to accept far less land than they have won through military action over the last 13 months. The Serbs now hold about 70% of the land in Bosnia and would retain just over 40% under the Vance-Owen plan.

There is no guarantee that extremist guerrilla units would go along. Posen noted that a few thousand Serbs could use sniper fire and terrorism to reignite full-scale war.

To Posen, the most unrealistic aspect of the plan is that it envisions the movement of Muslims and other ethnic groups to newly designated ethnic provinces, many of which are located between cantons that would be held by the Serbs. By some estimates, as many as 1 million people might move.

“As it’s written, the plan is unenforceable,” Posen contended.

Partly to help cope with these challenges, U.S. and allied military strategists were planning to use as much manpower and firepower as possible.

Under the aegis of the North Atlantic Treaty Organization, the United States had planned to put several thousand combat troops on the ground in Bosnia within a week after the Serbian parliament had ratified the plan--with plenty of air cover and permission virtually to fire at will.

Within a few days, the operation would have been expanded to include heavy armor, such as tanks and fighting vehicles, and weapons that can track and destroy enemy artillery and mortar emplacements within 30 seconds of their firing.

Robert W. Gaskin, a former Pentagon strategist who now is a vice president of Business Executives for National Security, a defense-related research group, said that the strategy is to mount an immediate show of force designed to discourage violations.

“They’re going to kick butt right off the bat,” Gaskin said.

Even so, retired Army Lt. Gen. Thomas W. Kelly, who was operations director for the Joint Chiefs of Staff during the 1990-91 Persian Gulf War, warned that there are enough extremists on all sides to make the dangers of any intervention very real.

And he said that the mission imposed by the Vance-Owen plan is so sweeping that allied troops are likely to be stretched thin. “If you want to keep a road open and you’ve got a guerrilla force that wants to close it, you have to put people along every foot of it,” Kelly said.