On May 13, 1990, in Zagreb, two of Yugoslavia’s biggest football teams, Red Star Belgrade and the home team, Dinamo Zagreb, were set to play in the Maksimir Stadium of the Croatian capital. Among the excited Red Star fans riding on the train to Zagreb that day was a young law student named Aleksandar Vucic. On the streets, fights began breaking out between rival fans. Cafe tables were flipped over, windows smashed. But the real ruckus erupted at the stadium, where Red Star fanatics, led by Serbian gangster Zeljko Raznatovic, aka Arkan, broke through the barrier holding them back and...