Alexander Jagiellończyk, b 5 August 1461 in Cracow, d 19 August 1506 in Vilnius. Son of King Casimir IV Jagiellończyk of Poland; grand duke of Lithuania (1492–1506), and Polish king (1501–6). During Alexander's reign Ivan III of Muscovy and the Crimean Tatars encroached on the Ukrainian territories belonging to Lithuania. In 1490–1500 Muscovy invaded and took control of the lands of Chernihiv and Novhorod-Siverskyi. In 1499 the Vilnius treaty was concluded, providing for the military union of Lithuania and Poland, which was then ruled by Alexander's brother, Jan I Olbracht, as equal states. During Alexander's rule Ukrainian magnates headed by Prince Mykhailo Hlynsky, a friend of the king, had considerable influence at the Lithuanian court. After Alexander's death the influence of Ukrainian magnates diminished.