Serbian and Hungarian leaders witness military showcase in Serbia.

The leaders of Serbia and Hungary attended a Serbian military exercise, which included low passes by Russian-made fighter jets and a display of a recently delivered Chinese anti-aircraft missile system and drones. Both leaders have maintained political and economic ties with Russia despite its aggression in Ukraine, which has been a sore spot with other European nations and with the U.S. Serbia's military buildup has raised concerns among some of the country's neighbors, which fear it could threaten the fragile peace in the region that was engulfed in a bloody war in the 1990s.