Poland-LithuaniaThe Polish–Lithuanian Commonwealth, formally the Kingdom of Poland and the Grand Duchy of Lithuania (also known as the Kingdom of Poland, or just 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 one of the most populous countries of 16th- and 17th-century Europe, with some 450,000 square miles (1,200,000 km2) and a multi-ethnic population of 11 million at its peak in the early 17th century. It was established at the Union of Lublin in July 1569, but the actual personal union between the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania had begun when Lithuania's Grand Duke Jogaila married Queen regnant Jadwiga of Poland and was crowned King jure uxoris Władysław II Jagiełło of Poland, in 1386. The Commonwealth was reduced in the First Partition of Poland in 1772 and disappeared as an independent state after the Third Partition of Poland in 1795.
The Union possessed 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. The two component states of the Commonwealth were formally equal, yet 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.
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 Constitution of May 3, 1791—the first codified constitution in modern European history and the second in modern world history (after the United States Constitution).