The Polish–Lithuanian Commonwealth, also referred to as Poland–Lithuania or the First Polish Republic (Polish: I Rzeczpospolita), was a federative real union between the Kingdom of Poland and the Grand Duchy of Lithuania, existing from 1569 to 1795. This state was among the largest, most populated countries of 16th- to 18th-century Europe. At its peak in the early 17th century, the Commonwealth spanned approximately 1,000,000 km2 (390,000 sq mi) and supported a multi-ethnic population of around 12 million as of 1618. The official languages of the Commonwealth were Polish and Latin, with Catholicism as the state religion.
The Union of Lublin established the Commonwealth as a single entity on 1 July 1569. The two nations had previously been in a personal union since the Krewo Agreement of 1385 (Polish–Lithuanian union) and the subsequent marriage of Queen Jadwiga of Poland to Grand Duke Jogaila of Lithuania, who was crowned as Władysław II Jagiełło, 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. It introduced an elective monarchy, whereupon members of domestic noble families or external dynasties were elected to the throne for life.
The Commonwealth's parliamentary system of government and elective monarchy, called the Golden Liberty, were an early example of constitutional monarchy. The General Sejm, the bicameral Parliament, held legislative power; its lower house was elected by szlachta nobles comprising some 10% of the population. A constitutional statute, the Henrician Articles, bound the king and his government, which tightly circumscribed royal authority. The country also exhibited unusual levels of ethnic diversity and great religious tolerance by European standards, guaranteed by the Warsaw Confederation Act of 1573, though the practical degree of religious freedom varied. Poland acted as the dominant partner in the union. Polonization of nobles was generally voluntary, but state efforts at religious conversion were sometimes resisted.
After a long period of prosperity, the Commonwealth found itself under sustained, combined assault from its neighbours and entered a period of protracted political and military 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 modern codified constitution in European history and the second in 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, Latin: Regnum Poloniae Magnusque Ducatus Lithuaniae). The Latin term was usually employed in international treaties and diplomacy. By concluding the 1569 Union of Lublin the Grand Duchy of Lithuania remained a separate state from the Crown of the Kingdom of Poland with its own name, laws, and territory. The name 'Commonwealth of Two Nations' (Polish: Rzeczpospolita Obojga Narodów) came into force during the reign of Stephen Báthory (since 1582, and was officially used until 1795). On 28 January 1588, Sigismund III Vasa confirmed the Third Statute of Lithuania in which it was stated that the Polish–Lithuanian Commonwealth is a federation of two countries – Grand Duchy of Lithuania and the Kingdom of Poland where both countries have equal rights within it.
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.
Western Europeans often simplified the name to 'Poland' and in most past and modern sources it is referred to as the Kingdom of Poland, or just Poland. The terms 'Commonwealth of Poland' and 'Commonwealth of Two Nations' (Polish: Rzeczpospolita Obojga Narodów, Latin: Res Publica Utriusque Nationis) were used in the Reciprocal Guarantee of Two Nations (1791). In the preamble of the Reciprocal Guarantee of Two Nations (1791) the dualistic nature of the state was confirmed and separate monarchial titles of Stanisław August Poniatowski were included: King of Poland (Polish: Krol Polski) and Grand Duke of Lithuania (Polish: Wielki xiązę litewski), also the land envoys of the Polish Crown and the Grand Duchy of Lithuania (Polish: posłów ziemskich Korony Polskiey, y Wielkiego Xięstwa Litewskiego) were mentioned. The English term Polish–Lithuanian Commonwealth and German Polen–Litauen are seen as renderings of the 'Commonwealth of Two Nations' variant.
Other informal names include the 'Republic of Nobles' (Polish: Rzeczpospolita szlachecka) and the 'First Polish Republic' (Polish: I Rzeczpospolita), the latter relatively common in historiography to distinguish it from the Second Polish Republic. In Lithuania, the state is referred to as 'Republic of Both Nations' (Lithuanian: Abiejų Tautų Respublika).
The Kingdom of Poland and the Grand Duchy of Lithuania underwent an alternating series of wars and alliances across the 13th and 14th centuries. The relations between the two states differed at times as each strived and competed for political, economic or military dominance of the region. In turn, Poland had remained a staunch ally of its southern neighbour, Hungary. The last Polish monarch from the native Piast dynasty, Casimir the Great, died on 5 November 1370 without fathering a legitimate male heir. Consequently, the crown passed onto his Hungarian nephew, Louis of Anjou, who ruled the Kingdom of Hungary in a personal union with Poland. A fundamental step in developing extensive ties with Lithuania was a succession crisis arising in the 1380s. Louis died on 10 September 1382 and, like his uncle, did not produce a son to succeed him. His two daughters, Mary and Jadwiga (Hedwig), held claims to the vast dual realm.
The Polish lords rejected Mary in favour of her younger sister Jadwiga, partly due to Mary's association with Sigismund of Luxembourg. The future queen regnant was betrothed to young William Habsburg, Duke of Austria, but certain factions of the nobility remained apprehensive believing that William would not secure domestic interests. Instead, they turned to Jogaila, the Grand Duke of Lithuania. Jogaila was a lifelong pagan and vowed to adopt Catholicism upon marriage by signing the Union of Krewo on 14 August 1385. The Act imposed Christianity in Lithuania and transformed Poland into a diarchy, a kingdom ruled over by two sovereigns; their descendants and successive monarchs held the titles of king and grand duke respectively. The ultimate clause dictated that Lithuania was to be merged in perpetuity (perpetuo applicare) with the Polish Kingdom; however, this did not take effect until 1569. Jogaila was crowned as Władysław II Jagiełło at Wawel Cathedral on 4 March 1386.
Several minor agreements were struck before unification, notably the Union of Kraków and Vilnius, the Union of Vilnius and Radom and the Union of Grodno. Lithuania's vulnerable position and rising tensions on its eastern flank persuaded the nobles to seek a closer bond with Poland. The idea of a federation presented better economic opportunities, whilst securing Lithuania's borders from hostile states to the north, south and east. Lesser Lithuanian nobility were eager to share the personal privileges and political liberties enjoyed by the Polish szlachta, but did not accept Polish demands for the incorporation of the Grand Duchy into Poland as a mere province, with no sense of autonomy. Mikołaj "the Red" Radziwiłł (Radvila Rudasis) and his cousin Mikołaj "the Black" Radziwiłł, two prominent nobles and military commanders in Lithuania, vocally opposed the union.
A fierce proponent of a single unified Commonwealth was Sigismund II Augustus, who was childless and ailing. According to historians, it was his active involvement which hastened the process and made the union possible. A parliament (sejm) convened on 10 January 1569 in the city of Lublin, attended by envoys from both nations. It was agreed that the merger will take place the same year and both parliaments will be fused into a joint assembly. No independent parliamentary convocation or diet was henceforth permitted. Subjects of the Polish Crown were no longer restricted in purchasing land on Lithuanian territory and a single currency was established. Whilst the military remained separate, a unified foreign policy meant that Lithuanian troops were obliged to contribute during a conflict not to their advantage. As a result, several Lithuanian magnates deplored the accords and left the assembly in protest. Sigismund II used his authority as grand duke and enforced the Act of Union in contumaciam. In fear, the absent nobles promptly returned to the negotiations. The Union of Lublin was passed by the gathered deputies and signed by attendees on 1 July, thus creating the Polish–Lithuanian Commonwealth.