Polish–Lithuanian Commonwealth

The Polish–Lithuanian Commonwealth (or Union, after 1791 the Commonwealth of Poland) was a dualistic state of Poland and Lithuania ruled by a common monarch. It was the largest and one of the most populous countries of 16th- and 17th-century Europe, with some 1,000,000 square kilometres (390,000 sq mi) 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 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 more powerful 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 written constitution in modern European history and the second in modern world history.