After the death of Casimir IV in 1492 the Lithuanian lords elected his son, Alexander, as grand duke in accordance with his father’s wishes. Casimir had bequeathed the Polish crown to his eldest son, Jan Olbracht. The personal union was suspended during Jan Olbracht’s reign, although, like his grandfather, he used the title of supremus dux in Lithuania. He formed a plan for an anti-Turkish coalition which, instead of dealing with the Turkish threat, led to conflict with the Moldavian prince, Stephen, and defeat at his hands. An agreement was struck with the Turks which was to last many years. The king’s success was the incorporation of the duchy of Plock in Mazovia into the Polish crown. In domestic affairs he relied on the support of the parliamentary chamber of deputies against the magnates of the royal council or senate. The short reign of Jan Olbracht provoked a counter-attack from the magnates during the reign of his brother Alexander (from 1501 onwards).
Alexander’s reign in Lithuania opened with the loss of Viaz'ma to Ivan III which led to a speedy strengthening of the alliance with Poland. The war with Moscow (1500—3) brought Lithuania losses beyond the Dnieper and the beginning of the division of Belorussian and Ukrainian lands between Lithuania and Muscovy. After his election to the Polish throne, Alexander was compelled to consent to a change in the form of government which henceforth placed decisions of the highest importance for the state in the hands of the full council known, since the sixteenth century, as the senate. The government of the oligarchy, which had control during the king’s absence on campaign against Moscow, provoked widespread opposition among the gentry. Relying on the advice of his chancellor, Jan Laski, the king renewed his collaboration with the chamber of deputies and introduced genuine fiscal and administrative reforms, including the law Nihilnovi (1505) which forbade the promulgation of new laws without the consent of the senators and the deputies of the region. This opened the way for a monarchia mixta on the Polish model with strong participation by the gentry. On Alexander’s death in 1506 the throne passed to his ‘only heir and successor’ in both Jagiellonian states, the last of the brothers, Zygmunt I (Sigismund) ‘the Old’.