For the first U.S. Marines rumbling into southeastern Kosovo this morning, it was a too-close encounter: Departing Yugoslav army armor nearly brushing against Marine armor, opposing troops eyeball to eyeball on a narrow country road.

"All it takes is one individual," said a nervous Lt. Col. Bruce Gandy, the Marine commander on the scene.

But as a Yugoslav armored personnel carrier passed the American convoy, its assault vehicles bristling with weaponry, it was the Marines who blinked -- in surprise. "Did you see that?" marveled Lance Cpl. Jason Brendt, a 25-year-old from Montana. "A giant stuffed Mickey Mouse in that Serb APC!"

The 900 Marines -- the first of 7,000 U.S. troops who will take part in the Kosovo peacekeeping operation -- have been assigned to one of the quietest sectors in the province but were taking no chances as they deployed on Day 3 of the NATO intervention. Concerned that French NATO forces were still untangling columns of Yugoslav military troops and equipment pulling out of the region, Marine commanders ordered the detachment to spend the night in a wheat field near Pozaranje, a village in southeastern Kosovo about 10 miles from their intended destination.

The Marines entered southeastern Kosovo in a high-alert, high-caution wartime stance that contrasted sharply with the demeanor of British armored calvary troops who led the NATO peacekeeping contingent into Kosovo two days earlier. Whereas the British reveled in the outpouring of emotion by crowds of ethnic Albanians who greeted them with tears, chants and bouquets of fresh flowers, the Marines saw danger.

"It was very distracting," said Staff Sgt. Vincent Lovitt, 40, of Quantico, Va. "We were supposed to be watching the buildings for snipers."

Even the wheat field, set amid a rural landscape that could have been a backdrop for a Grandma Moses painting, was suspect. An enormous plow designed to unearth land mines spent the afternoon chewing vast swaths out of the pastel-green grass before Marines in several dozen vehicles were permitted to set up camp.

When NATO divided Kosovo into sectors to be controlled by different countries, U.S. Marine and Army forces were alotted the eastern portion of the province around the town of Gnjilane, the region perhaps least affected in recent months by the ravages of war. While most villages remain deserted and looted, few houses were burned or shelled, and little fighting occurred here.

NATO military leaders hope to prevent the kind of looting and pillaging by departing Yugoslav forces that preceded the arrival of the first British NATO forces this weekend, Marine commanders said, by leaving no gap between the exit of Yugoslav troops and the entry of peacekeepers. But that has led to the unwieldy entanglement of arriving NATO troops and the departure of NATO-escorted Yugoslav forces, making U.S. commanders -- perhaps the most security conscious of all NATO officers here -- all the more cautious.

But the Yugoslav and Serbian forces leaving the region -- escorted by French troops assigned to clear out the region before the Americans moved in -- offered no obvious resistance today. In fact, as they passed the incoming Marines, "they just smiled and waved," said Lance Cpl. Erik Casteneda, 20, of Hoffman Estates, Ill., who watched them from atop his armored vehicle.

The Marines moving into this area bring a perspective to the job that some of the other peacekeepers do not. While they may be assigned to one of the least contentious areas of the province, for more than a month they were assigned to a refugee camp in southern Albania, where owners of many of the deserted homes here sought shelter and are now longing to return.

CAPTION: Ethnic Albanians cheer and toss flowers as a convoy carrying the first detachment of 900 U.S. Marines rolls into Kosovo, where it will occupy southeastern sector.

CAPTION: Marine Cpl. Anthony Arnetta of Baltimore receives flowers and a hug from an ethnic Albanian child upon arriving in Kosovo. Thousands shouted "NATO USA" as the American troops rolled into Pozaranje.