Privilege of Mielnik

The Privilege of Mielnik (Polish: Przywilej mielnicki) was an act granted on October 25, 1501, at Mielnik by Poland's King Alexander Jagiellon. It substantially expanded the powers of the Senate and the magnates, at the expense of the king. Furthermore, it de facto introduced a republican form of government. The privilege was, however, not confirmed by the king after his election, and in 1504 was rejected by the Sejm (see Łaski's Statutes).

After the death of Kazimierz Jagiellonczyk (1492), the personal union between the Kingdom of Poland and the Grand Duchy of Lithuania was broken. New king of Poland was Jan Olbracht, while Lithuania was governed by his brother Aleksander. Following his father, Olbracht tried to win the support of the nobility, granting it several concessions in the Statutes of Piotrków. The failed invasion of Moldavia in 1497 complicated internal situation of Poland, and strengthened the position of the magnates. At the same time, Lithuania was attacked by Ivan III of Russia, and in 1499 both nations signed the Union of Kraków and Vilna, which returned the Polish – Lithuanian alliance. In 1500 Jan Olbracht died, and on October 3, 1501, Aleksander Jagiellonczyk became the King of Poland. His election worsened the position of the Lithuanians, who agreed to the Union of Mielnik. Aleksander himself had to sign the Privilege of Mielnik, which limited his authority, and expanded the powers of the Senate:

...
Wikipedia