Following Murad's death at the Battle of Kosovo, however, the Anatolian beyliks sensed an opportunity to restore their fortunes and formed an anti-Ottoman league under the Karamanid ruler Ala ad-Din Ali (r. 1361–1398): the Karamanids advanced up to Eskişehir, the Germiyanid ruler Yakub II restored his principality, and Burhan al-Din took Kırşehir.[3] As a result, Murad's son and successor Bayezid I (r. 1389–1402) crossed the Dardanelles with his own forces, supported by the armies of the Jandarid Süleyman Pasha of Kastamonu and the Christian vassal states of the Balkans—mainly Serbia and the Byzantine Empire—and launched a campaign to "unify Anatolia under Ottoman rule".[4][5] Much of the information about Bayezid's campaigns in 1390–1391 comes from the letters written by the Byzantine emperor Manuel II Palaiologos, who accompanied Bayezid as his vassal.[6]