The Polish-Lithuanian Commonwealth, also known as the "Republic of the Two Nations" or "Commonwealth of the Two Nations" ( Polish: Rzeczpospolita Obojga Narodów; Belarusian: Рэч Паспалітая Абодвух Народаў; Lithuanian: Abiejų tautų respublika) or as the "First Republic," was one of the largest and most populous countries in 17th-century Europe. Its political structure — that of a semi-federal, semi-confederal aristocratic republic — was formed in 1569 by the Union of Lublin, which united the Kingdom of Poland and the Grand Duchy of Lithuania, and lasted in this form until the adoption of the Constitution of May 3, 1791. The Commonwealth covered not only the territories of what is now Poland and Lithuania, but also the entire territory of Belarus and Latvia, large parts of Ukraine and Estonia, and part of present-day western Russia ( Smolensk oblast).