How do nationalistic media affect animosity between ethnic groups? We consider one of Europe's deadliest conflicts since WWII, the Serbo-Croatian conflict. We show that, after a decade of peace, cross-border nationalistic Serbian radio triggers ethnic hatred toward Serbs in Croatia. Mostly attracted by nonpolitical content, many Croats listen to Serbian public radio (intended for Serbs in Serbia) whenever signal is available. As a result, the vote for extreme nationalist parties is higher and ethnically offensive graffiti are more common in Croatian villages with Serbian radio reception. A laboratory experiment confirms that Serbian radio exposure causes anti-Serbian sentiment among Croats.