According to Matteo Villani, Louis launched an expedition against the Golden Horde at the head of an army of 200,000 horsemen in April 1354. The young Tatar ruler, whom historian Iván Bertényi identified as Jani Beg, did not want to wage war against Hungary and agreed to sign a peace treaty. Although no other primary source mentioned that campaign and treaty, the Tatars made no plundering raids in Transylvania after 1354, which suggests that Villani's report is reliable. In the same year, Louis invaded Serbia, forcing Stefan Dušan to withdraw from the region along the river Sava. Under duress, Dušan initiated negotiations with the Holy See for acknowledgement of the popes' primacy. The following year, Louis sent reinforcements to Casimir III to fight against the Lithuanians, and Hungarian troops supported Albert II, Duke of Austria, against Zürich. The Venetian delegates offered Louis 6–7,000 golden ducats as a compensation for Dalmatia, but Louis refused to give up his plan to reconquer the province. He signed an alliance with Albert II of Austria and Nicolaus of Luxemburg, Patriarch of Aquileia, against Venice. Upon his order, Croatian lords besieged and captured Klis, a Dalmatian fortress that Stefan Dušan's sister, Jelena, had inherited from her husband, Mladen Šubić.