THE United States and its NATO allies are gearing up for a mission in Bosnia-Herzegovina unprecedented in military history: keeping rival armies apart long enough to allow the return of peace to a country riven by almost four years of ethnic war.
To accomplish that goal, the 60,000 members of the US-led Implementation Force, or IFOR, will have to surmount major challenges. Not only might they have to fight those who oppose the peace deal reached on Nov. 21 in Dayton, Ohio, but they also face other threats ranging from disgruntled war-profiteers and millions of snow-covered land mines to frostbite and muddy mountain roads.
The men and women of IFOR also will have to deal with maintaining morale, professionalism, and evenhandedness as they suffer casualties defending lands and people not their own.
''Nobody has ever attempted anything like this before,'' says Paul Beaver of Jane's Defense Week in London, noting that this is a peace-enforcing, not peacekeeping mission. ''It is going to require a tremendous amount of cooperation among the 20 or so nations involved. It's also going to require a lot of ... support, and politicians back home are going to have to understand that there are going to be casualties.''
President Clinton says Operation Joint Endeavor, NATO's largest-ever mission, will achieve its goal of securing within a year the conditions for elections, reconstruction, and the beginning of a long, difficult reconciliation between Bosnia's Muslims, Serbs, and Croats.
''In Bosnia, we can and will succeed because our mission is clear and limited,'' Mr. Clinton said Monday in his television address.
Some experts, however, are deeply skeptical. They worry that a premature NATO departure could reignite fighting that could spread into other parts of the Balkans.
''The time frame terrifies me. It simply isn't enough,'' says Michael Williams of London's Institute for International and Strategic Studies and the former spokesman for the UN mission in former Yugoslavia.
Most experts say the main threat will be extremist elements opposed to the peace deal. This applies particularly to the Bosnian Serbs, many of whom object to the awarding of the now-divided capital, Sarajevo, to the Muslim-Croat federation, and a denial of the right of secession.
US officials are counting on President Slobodan Milosevic of Serbia, the Bosnian Serbs' erstwhile political master, to whip his proxies into line with the accord he signed on their behalf in Dayton. But there are no reliable signs that Bosnian Serb leader Radovan Karadzic or his military boss, Gen. Ratko Mladic, are ready to cooperate.
Mr. Karadzic and General Mladic are barred by the accord from running for office because they have been indicted by the UN War Crimes Tribunal. So has Dario Kordic, the Bosnian Croat political leader. Efforts to arrest them could trigger retaliation against IFOR.
Clinton officials warn that any attack on IFOR will be met with a disproportionate response. But European officials and analysts question whether the pact could survive a major clash between IFOR and one of the Bosnian sides. It was a reluctance to cross this line that contributed to the humiliation of the departing UN peacekeeping force, they explain.
IFOR ''may end up facing many of the difficulties that the UN did,'' says Mr. Williams. ''Sure, the rules of engagement are different, and [IFOR is] armed to teeth. But they are still meant to be evenhanded.''
A reconnaissance team of 10 US soldiers arrived in the northeastern Bosnian city of Tuzla yesterday, making way for an advanced guard of 1,500 IFOR soldiers, including up to 700 Americans, who could arrive in Bosnia as early as this weekend. More troops would begin moving within hours of the formal signing in Paris in mid-December of the Dayton peace agreement.
IFOR is to include 20,000 US troops, drawn mostly from the 1st Armored Division, based in Germany. They are to be deployed in northeastern Bosnia, with their headquarters in Tuzla. Poland, the Baltic states, and Russia will also contribute troops. IFOR's other major components will be some 15,000 British soldiers and 10,000 French troops who are already in Bosnia as UN peacekeepers. IFOR will be led by US Adm. Leighton Smith, chief of NATO's southern command.
IFOR will ensure that the Bosnian armies withdraw from front lines within 30 days and then deploy in the resulting ''zone of separation.'' It will also enforce a 45-day deadline for land transfers between the Muslim-Croat federation and the Bosnian Serbs.
Within four months of the Paris signing, the Bosnian armies will have to withdraw all men and weapons to their bases and provide inventories and locations to IFOR. In addition to ensuring the military disengagement, the NATO force will help move humanitarian supplies and assist in the return and protection of refugees.
IFOR has the power to compel compliance with the peace pact by force if any side fails to do so. It also has what US officials call a ''silver bullet'' - the right to take pre-emptive action against soldiers or weapons deemed a threat.
Many experts say the US contingent will have an easier task than its IFOR partners because there are few areas of contention in its sector. One possible source of trouble is Bosnian Serb guns that target Tuzla. A single shell killed 72 people there earlier this year.
Getting troops into position will also be a challenge in the harsh Balkan winter. Many bridges are damaged and unsafe for armored vehicles, roads are icy, and an estimated 6 million land mines are covered by snow. Most of the 214 deaths of UN peacekeepers resulted from land mine and road accidents.
The US itself could create problems with its plan to arm and train the Muslim-led Bosnian Army. US officials say NATO can withdraw only if it creates a military counter-weight to the more powerful Bosnian Serb force. But some experts say the plan could stir retaliation against US troops.