NEWS ANALYSIS : ‘Blue Helmets’ Face Most Perilous Peacekeeping Challenge in Croatia : Yugoslavia: Fierce resistance anticipated as they attempt to disarm whole communities. They remain distrusted by militants on all sides.

Rested up and with swords sharpened, the combatants in the Yugoslav civil war are poised after a six-week lull in fighting to make the biggest United Nations peacekeeping mission in 30 years one of its most dangerous.

When 13,000 U.N. “Blue Helmets” begin deploying to Croatian war zones next month, they will encounter fierce resistance as they attempt to disarm whole communities that are convinced that true power travels through the barrel of a gun.

The Balkan war that has killed 10,000--many of them civilians--and driven 750,000 people from their homes is a nationalist conflict of local making. Many in the clannish, backward regions where the worst of the clashes have erupted firmly believe that foreign troops have no business butting in.

U.N. forces are pledged to remain neutral. Nevertheless, they are distrusted by militants on all sides.

Worsening the prospects for a peaceful intervention is the United Nation’s reversal of earlier pledges to stay out unless the world body’s assistance was unanimously requested. One Serbian warlord continues to openly reject the U.N. plan to end the Balkan bloodshed; others are believed to share his view.

Milan Babic, leader of ethnic Serbs in southwestern Croatia, warned the United Nations in a letter disclosed on Friday that peacekeepers risk “many casualties” if they trespass on his turf.

The decision to urge deployment suggests the peacekeeping administrators are convinced they can isolate Babic, likely based on assurances from Serbia that it will cut off weapons and supplies.

But Babic is not alone in opposing the U.N. plan.

The federal army’s commitment to withdraw from the conflict is suspect, as are the motives of the Serbian and Croatian presidents in appealing to the United Nations to halt the ethnic fighting they sparked.

The U.N. troops, once rounded up and equipped for indefinite deployment, also may find that the window of opportunity for intervention has closed. During six relatively peaceful weeks when Croatian national guardsmen and Serbian-led federal troops have tried to maintain a cease-fire, U.N. officials gave out mixed signals about whether they would, in the end, intervene. It was only when the truce began visibly unraveling over the last few days that Cyrus R. Vance, the U.N. special envoy to Yugoslavia and former secretary of state, urged deployment.

At Vance’s suggestion, U.N. Secretary General Boutros Boutros-Ghali on Thursday recommended deployment to the 15-nation Security Council, which is expected to endorse the proposal at a meeting in New York next week.

The Yugoslav peacekeeping force would be the largest since 1960, when 20,000 U.N. troops were sent to the Belgian Congo, now Zaire. It also would be the most costly, at an estimated $400 million a year; the force would be the first such U.N. peacekeeping contingent deployed in continental Europe.

More than 500,000 soldiers of various nationalities have served under the U.N. flag over the last four-plus decades, with nearly 800 of them killed in the line of duty. The Yugoslav war has posted tolls higher than that in a single month, and some guerrilla factions have threatened to shoot at U.N. troops.

Because no diplomatic solution to the Balkan crisis is on the horizon, the length of the U.N. mission is difficult to predict. Croatian officials want it limited to six months and are banking on U.N. forces helping them recover territory now occupied by Serbs and the army.

Boutros-Ghali reportedly wants a one-year mandate for the U.N. forces. But how they would extricate themselves if there still were no negotiated settlement is unclear.

Balkan tensions simmered for more than a decade before bursting into armed conflict last June, and resolution of the increasingly complex issues propelling the war would be unlikely within 12 months. If it could be accomplished amid the pitfalls, intervention of the lightly armed peacekeepers could initially deter major outbreaks of fighting, as the leaders of Serbia, Croatia and the Yugoslav army have promised to comply.

But with millions of weapons in the hands of civilians and militant factions within all three forces itching to finish the fight, introduction of another armed contingent could produce more risk than reward. “The Blue Helmets will take some casualties. There’s no doubt about that,” said a Western diplomat in Belgrade. “But that doesn’t mean they shouldn’t come in. If they don’t move soon, there won’t be any peace left to keep.”

Fighting has surged in recent days, especially in the eastern Croatian area around the city of Osijek, causing the number of casualties to creep back toward the daily tolls suffered late last year. Vance was also concerned that the conflict could spread to other Balkan areas, particularly to the republics of Macedonia and Bosnia-Herzegovina. They have asked for but not yet received international recognition of their statehood.

Bosnia-Herzegovina now is occupied by tens of thousands of federal troops loyal to Serbia, which has designs on much of the republic’s territory. Macedonia is being economically strangled by blockades imposed by Serbia and Greece, both aimed at deterring independence.

Serbian President Slobodan Milosevic and Croatian leader Franjo Tudjman now seem genuinely to want U.N. intervention, but their motives appear political. They were largely responsible for provoking the war but both have seen their power bases eroded in recent months as people have been stunned by the mounting casualties and deepening poverty.

The degree of support for U.N. intervention among forces of the Yugoslav Peoples Army is difficult to gauge. Gen. Blagoje Adzic, the new federal defense minister, has said he would abide by orders from the Serbian-controlled federal presidency, which is staffed with staunch Milosevic allies and has, therefore, given all appearances of support for the U.N. mission.

But the army would be required to withdraw from Croatia under the U.N. plan. That would effectively limit the army’s mandate to defending Serbia and Montenegro, which are too poor to support the massive, pampered military force. The 180,000-member Yugoslav army, one of the largest in Europe, intervened in the war on behalf of Serbian rebels in hopes of preserving a Yugoslav state large enough to support it.

Without Croatia’s naval ports and broad access to the Adriatic Sea, the federal navy is forced to put in at tiny Montenegro’s sole deep harbor, Kotor Bay. Slovenia and Croatia, both of which have seceded, provided about half of the former Yugoslavia’s tax revenue, so their independence has severely depleted the federal coffers.

Besides the economic incentive for the army to continue fighting, Adzic is a renowned Communist hard-liner whose entire family was slaughtered by Croatian fascists during World War II; that makes it unlikely that he would abandon the cause of defending fellow Serbs in other republics.