ZVECAN, Kosovo — In the densely forested mountains along the contested frontier between Serbia and Kosovo, a patrol of American soldiers under NATO command trudged through snow and mud, keeping an eye out for smugglers or anyone else trying to cross the border. Given the bloody legacy of this area, the situation is quiet now, at least up here.

It is down below, in Serbia and Kosovo, where old angers are resurfacing as the Balkan region that spawned so much suffering over the last century is again becoming dangerously restive. And once again, Russia is stoking tensions, as it seeks to exploit political fissures in an area that was once viewed as a triumph of muscular American diplomacy — but that now underscores the growing challenges facing NATO and the European Union.

“Russia sees the West meddling in its backyard, and President Vladimir V. Putin wants to show he can reciprocate,” said Dimitar Bechev, an expert on Russia and the Balkans and head of the European Policy Institute in Sofia, Bulgaria. “They see the Balkans as the West’s underbelly, and they use it to throw their weight around and project power on the cheap.”

Nearly 18 years after a United States-led intervention ended Serb domination of Kosovo, the border patrols are part of the longest-running mission in NATO history. Even as the European Union has made limited progress in brokering a political settlement between Kosovo and Serbia, the presence of NATO forces has maintained an uneasy peace, with animosity between the minority Serbs and majority Albanian inhabitants of Kosovo still palpable.