Emptying the Yugoslav province of Kosovo of half its population of ethnic Albanians required roughly two months' work by 40,000 Yugoslav troops and Serbian police. But enabling these refugees to return to their homes and remake their lives will require billions of dollars of Western aid and deployment of a peacekeeping force that will likely be at least one-third larger than the Serb-led force and remain in Kosovo for many years.

Just to begin the effort, NATO must overcome a host of political and logistical challenges. It must negotiate the timing and specifics of the withdrawal of Yugoslav forces as well as the rules governing activities by hundreds of government troops that can return to Kosovo. U.S. and Yugoslav military officers are slated to hold preliminary talks Saturday morning near the Blace border crossing between Kosovo and Macedonia where hundreds of thousands of refugees fled in terror.

The next steps will be no easier. The difficulties illustrate lessons that diplomats and military officers say they gleaned from international efforts to resolve conflicts earlier in the decade in Bosnia and Croatia: Waiting to resolve disputes exacts a large cost. Destruction is easier and more swift than rebuilding. Renewal is rarely accomplished efficiently or completely.

NATO forces, U.N. officials and aid workers must persuade more than 800,000 refugees that they will be safe if they return. Then the international workers must attempt to impose order and discipline on the refugees' yearnings to see what remains of their possessions, track down missing family members and friends and restart their lives.

When NATO troops establish the first Western military beachhead in Kosovo, possibly within a few days, they will face peril from thousands of mines and a possible threat from Serb paramilitary units or armed civilians. The residents who come back weeks and months later may have to face booby traps and poisoned well water, or find their lives disrupted by inadequate public services and weak local government.

"This is going to be a heck of a job," said Spanish diplomat Carlos Westendorp, who has overseen the reconstruction of neighboring Bosnia as U.N. representative there for the past two years. "The houses have been destroyed and the cattle have been slaughtered. The return of the refugees is going to be very hard. . . . Many experts say it is going to be easier to do this by winterizing the present camps" in Macedonia and Albania and letting only half of the refugees return this year.

Westendorp also said he has heard civilian estimates of the cost of rebuilding Kosovo in the range of $2.5 billion, or as much as half the amount spent in Bosnia -- a territory more than four times larger -- in the past four years. He added that the effort will face many potential pitfalls if NATO and other foreign powers fail to fully protect Kosovo. In particular, there's the prospect of continued tensions with the Yugoslav government as long as President Slobodan Milosevic remains in power.

"In Bosnia . . . we have a light civilian administration," a circumstance that has encouraged conflicts and slowed the process of repatriating refugees, Westendorp said. The approach "we need in Kosovo is a heavy and quick one" because each month that passes after the end of ethnic conflict makes the return of refugees more problematic.

To get as early a start as possible, NATO has placed some of the estimated 16,000 troops stationed in Macedonia on 24-hour alert for deployment in Kosovo. They are stationed now near the airport in Skopje, the capital, and can reach the border in an hour-long drive that takes them past two camps holding an estimated 30,000 refugees.

As these forces move north, they will be replenished by thousands of additional troops to be flown into the airport here in Thessaloniki. Military trucks, armored vehicles and engineering equipment are to be unloaded at piers in this Aegean seaport for duty in Kosovo. But the Greek government halted such arrivals last week to avoid disrupting European parliament elections on June 13, and has not said when it will lift the ban.

Some NATO and humanitarian officials worry that embittered Yugoslav troops will destroy more homes and poison wells as they leave the province. The pattern was seen during a NATO-mandated withdrawal from Kosovo last October, apparently with the aim of complicating the return of those who had been displaced by fighting.

What the troops will find in Kosovo is "a territory devoid of administration," because most of the Serbs who have governed the province have left already or likely will flee to avoid the "settling of accounts" when Yugoslav troops withdraw, said Alex Rondos, the Greek Foreign Ministry's top humanitarian aid adviser. "There will be no one to run the schools, the hospitals, the waterworks," unless the international community assumes responsibility for the province's governance, he said.

Rondos, whose colleagues have organized more than 20 convoys of aid inside Kosovo since the war began on March 24, said the Yugoslav government's destruction of towns and village has been selective. In a triangle of territory bounded by the city of Pec to the west, Prizren to the south and Pristina to the east -- where fighting was particularly heavy between government forces and members of the rebel Kosovo Liberation Army -- the destruction of houses has been "very serious," he said.

But elsewhere, Rondos said, individual houses have been destroyed, but not entire villages, which will complicate the task of deciding which refugees should be returned first. He said refugees probably should be told that no one will be able to go back for at least a month while such matters are sorted out. "It would be criminal not have fresh water, food supplies and some medicine" in place before they return, he said.

One challenge will be to ensure that those refugees who do return feel safe, a task that may be complicated by the presence of Yugoslav and Serbian forces at border crossings and near key monuments in Kosovo, as the agreement allows. A senior U.S. official recently expressed doubt that this arrangement will prove workable, noting that "if the [Yugoslav] forces are big enough to protect themselves, they are too big to be in Kosovo; if they are not big enough, then they are almost certainly excessively vulnerable" to attack.

Another challenge will be to reach an alliance consensus on whether and to what degree international aid will go to Yugoslavia. U.S. officials have said that no assistance will be given until Belgrade begins democratic reforms. But other NATO members -- including Greece, France, and Italy -- are expected to propose assistance to targeted groups and projects inside Yugoslavia, even before any change in the government.

CAPTION: A young ethnic Albanian refugee flashes a "V" sign during a children's patriotic song and poetry show at a camp on the outskirts of Kukes, Albania. NATO is seeking to return more than 800,000 Kosovo Albanians displaced during war.