Casimir III laid siege to Belz and Louis joined his uncle in March 1352. During the siege, which ended without the surrender of the fort, Louis was heavily injured in his head. Algirdas, Grand Duke of Lithuania, hired Tatar mercenaries who stormed into Podolia, Louis returned to Hungary because he feared a Tatar invasion of Transylvania. Pope Clement proclaimed a crusade against the Lithuanians and the Tatars in May, authorizing Louis to collect a tithe from Church revenues during the next four years. The pope stated that he had never "granted a tenth of such duration", emphasizing the link between his magnanimity and the release of the imprisoned Neapolitan princes. The pope also authorized Louis to seize the pagans' and schismatics' lands bordering on his kingdom.