The Polish–Lithuanian Commonwealth, formally the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania, after 1791 the Commonwealth of Poland, was a dualistic state, a bi-confederation of Poland and Lithuania ruled by a common monarch, who was both the King of Poland and the Grand Duke of Lithuania. It was one of the largest and most populous countries of 16th- and 17th-century Europe. At its peak in the early 17th century, the Commonwealth spanned almost 400,000 square miles (1,000,000 km2) and sustained a multi-ethnic population of 11 million.
The Commonwealth was established by the Union of Lublin in July 1569, but the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania had been in a de facto personal union since 1386 with the marriage of the Polish queen Hedwig and Lithuania's Grand Duke Jogaila, who was crowned King jure uxoris Władysław II Jagiełło of Poland. The First Partition of Poland in 1772 and the Second Partition of Poland in 1793 greatly reduced the state's size and the Commonwealth collapsed as an independent state following the Third Partition of Poland in 1795.
The Union possessed many features unique among contemporary states. Its political system was characterized by strict checks upon monarchical power. These checks were enacted by a legislature (sejm) controlled by the nobility (szlachta). This idiosyncratic system was a precursor to modern concepts of democracy, constitutional monarchy, and federation. Although the two component states of the Commonwealth were formally equal, Poland was the dominant partner in the union.
The Polish–Lithuanian Commonwealth was marked by high levels of ethnic diversity and by relative religious tolerance, guaranteed by the Warsaw Confederation Act 1573; however, the degree of religious freedom varied over time. The Constitution of 1791, however, acknowledged Catholicism as the "dominant religion," unlike the Warsaw Confederation; although, freedom of religion was still granted with it.
After several decades of prosperity, it entered a period of protracted political, military and economic decline. Its growing weakness led to its partitioning among its neighbors, Austria, Prussia and the Russian Empire, during the late 18th century. Shortly before its demise, the Commonwealth adopted a massive reform effort and enacted the May 3 Constitution—the first codified constitution in modern European history and the second in modern world history (after the United States Constitution).
The official name of the state was The Kingdom of Poland and the Grand Duchy of Lithuania (Polish: Królestwo Polskie i Wielkie Księstwo Litewskie, Lithuanian: Lenkijos Karalystė ir Lietuvos Didžioji Kunigaikštystė, Latin: Regnum Poloniae Magnusque Ducatus Lithuaniae) and the Latin term was usually used in international treaties and diplomacy. In the 17th century and later it was also known as the Most Serene Commonwealth of Poland (Polish: Najjaśniejsza Rzeczpospolita Polska, Latin: Serenissima Res Publica Poloniae), the Commonwealth of the Polish Kingdom, or the Commonwealth of Poland. Its inhabitants referred to it in everyday speech as the "Rzeczpospolita" (Ruthenian: Рѣч Посполита Rech Pospolita, Lithuanian: Žečpospolita). Western Europeans often simply called it Poland and in most past and modern sources it is referred to as the Kingdom of Poland, or just Poland. The terms: the Commonwealth of Poland and the Commonwealth of Two Nations (Polish: Rzeczpospolita Obojga Narodów, Latin: Res Publica Utriusque Nationis) were used in the Reciprocal Guarantee of Two Nations. The English term 'Polish–Lithuanian Commonwealth' and German 'Polen-Litauen' are seen as renderings of the Commonwealth of Two Nations variant. Other names include the Republic of Nobles (Polish: Rzeczpospolita szlachecka) and the First Commonwealth (Polish: I Rzeczpospolita), the latter relatively common in Polish historiography.
Poland and Lithuania underwent an alternating series of wars and alliances during the 14th century and early 15th century. Several agreements between the two (the Union of Kraków and Vilna, the Union of Krewo, the Union of Wilno and Radom, the Union of Grodno, and the Union of Horodło) were struck before the permanent 1569 Union of Lublin. This agreement was one of the signal achievements of Sigismund II Augustus, last monarch of the Jagiellon dynasty. Sigismund believed he could preserve his dynasty by adopting elective monarchy. His death in 1572 was followed by a three-year interregnum during which adjustments were made to the constitutional system; these adjustments significantly increased the power of the Polish nobility and established a truly elective monarchy.