"U boj, u boj" (Croatian: "To battle, to battle") is a Croatian patriotic song. It was written by Franjo Marković and composed by Ivan Zajc in 1866, who later incorporated it as an aria into his opera Nikola Šubić Zrinski (1876) where it is sung by a male choir.
The choir version of the song enjoys a wide popularity in Japan, where for a long time it was thought to be a Czech folk song. After the end of World War I, an American ship Hefron, evacuating Czech and Slovak soldiers from Siberia, amongst them this Croat song was wide popular during the World War One, was damaged by a storm and was forced to settle in the Kobe harbour for 2 months for repair. During that period, the soldiers transmitted the tune to the members of the oldest and the most renowned Japanese male choir of the Kwansei Gakuin University, established in 1899. Their repertoire hosts the song (which is learned not in Japanese, but in genuine Croatian) to this day, ending with it each of their public performances.
The lyrics "U boj, u boj" can also be heard during sports events in which Croatian national teams participate. Croatian supporters version has also second verse "Za narod svoj!", so in complete it goes: "To the fight, to the fight, for your people/nation"