"U boj, u boj" (Croatian for "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. It is a retelling of the Battle of Szigetvár of 1566, in which Nikola IV Zrinski, Ban of Croatia and captain of the assembled Croatian and Hungarian forces, took a heroic last stand against overwhelming Ottoman forces, led personally by Suleiman the Magnificent. Though the fortress fell, the defenders inflicted grievous injuries on the assaulting forces, all but crippling the victors' ability to progress past the Croatian-Hungarian border, and causing the death of the sultan himself.

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, the American ship Heffron, evacuating Czech and Slovak soldiers from Siberia, among whom this Croat song was widely popular during the war, was damaged by a storm and was forced to settle in the Kobe harbour for 2 months for repair. During that period, the soldiers passed on 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.