Though Serbia has an official policy of military neutrality vis-a-vis NATO, there's been increased cooperation over the past few years. However, both the Chinese drone acquirement and Present's Aleksandar Vucic recently signaling he'd like to acquire the S-400 anti-air defense system from Russia has caused Washington to threaten sanctions.
The delivery makes Serbia the first European country in history to deploy Chinese combat drones and thus is raising eyebrows in Brussels, as Bloomberg underscores:
China's actions are prodding the North Atlantic Treaty Organization to pivot to Asia, a potential sea change that's roiling an alliance that was created to protect Europe against the Soviet Union and then Russia. China's growing influence in the Balkans mirrors its push into other areas previously dominated by Russia. The Belt and Road enterprise already has made it a major player in Central Asian politics.