Polish–Lithuanian Commonwealth

The Polish–Lithuanian Commonwealth, formally known as the Kingdom of Poland and the Grand Duchy of Lithuania, or simply Poland–Lithuania, was a bi-confederal state, sometimes called a federation, of Poland and Lithuania ruled by a common monarch in real union, who was both King of Poland and Grand Duke of Lithuania. It was one of the largest and most populous countries of 16th- to 17th-century Europe. At its largest territorial extent, in the early 17th century, the Commonwealth covered almost 1,000,000 km² (400,000 sq mi) and as of 1618 sustained a multi-ethnic population of almost 12 million. Polish and Latin were the two co-official languages, and Roman Catholicism served as state religion.

The Commonwealth was established as a single entity by the Union of Lublin on 1 July 1569. The two nations had previously been in a personal union since the Krewo Agreement of 1385 and the subsequent marriage of Queen Jadwiga of Poland to Grand Duke Jogaila of Lithuania, who was crowned jure uxoris King of Poland. Their descendant, Sigismund II Augustus, enforced the merger to strengthen frontiers of his dominion and maintain unity as he remained childless. His death in 1572 marked the end of the Jagiellonian dynasty and introduced an elective monarchy, whereupon members of domestic noble families or external dynasties were elected to the throne for life.

The state possessed an idiosyncratic system of governance and its Golden Liberty placed controls upon monarchical authority, a precursor to modern concepts of democracy. Legislation was enacted by the General Sejm, a bicameral legislature (parliament) administered by the szlachta nobility, and the king was bound to comply with the constitutional principles dictated by the Henrician Articles. The country also maintained unprecedented levels of ethnic diversity and relative religious tolerance, guaranteed by the Warsaw Confederation Act of 1573, though the degree of religious freedom was not always uniform and varied over time. Poland acted the dominant partner in the union and Polonisation as well as Catholicism were imposed across the large realm, which was met with resistance from some minorities.

After several decades of prosperity, the Commonwealth entered a period of protracted political, military, and economic decline. Its growing weakness led to its partitioning among its neighbours, Austria, Prussia, and Russia, during the late 18th century. Shortly before its demise, the Commonwealth adopted a major reform effort and enacted the 3 May Constitution, which was the first codified constitution in modern European history and the second in modern world history after the United States Constitution.