The Polish–Lithuanian Commonwealth was a formal treaty imposed upon Lithuania to replace the loose personal union of the Crown of the Kingdom of Poland and that of the Grand Duchy of Lithuania, with a single political union with an elective monarchy.
Between November 1563 and June 1564, when it seemed a Muscovite invasion was likely, the worried Lithuanians attended a Polish Diet at Warsaw where an absolute union between the two countries, which Poland had been attempting to engineer for over twenty years, was hotly debated. There was deadlock, and the Polish King, Sigismund II, tactfully intervened and voluntarily relinquished his hereditary title of Grand Duke to Lithuania, thus placing the two countries on a constitutional equality. On 10 January 1569 the Lithuanians were summoned to another Diet convened by Poland, in Lublin, but even then the Lithuanians were indisposed towards a union and finally walked out of the Diet, leaving two commissioners as observers only. A frustrated Sigismund then invaded the Lithuanian provinces of Volhynia and Podolia and annexed them to the Kingdom of Poland. The Lithuanians were forced to return to the table as, they conceded, a union was better than mutilated independence. The union was adopted on July 1, 1569 as the Union of Lublin between the Kingdom of Poland and the Grand Duchy of Lithuania and was accordingly sealed. Only the Polish currency was permitted to be used, and all dependencies and colonies, including Livonia which had not long been ceded to Lithuania, were to belong to Poland in common. Essentially Lithuania, once the greater power with a greater land mass, had been eclipsed and absorbed by her neighbour. At this Lublin Diet the home rule of Lithuania also definitely ended; there was to be one Parliament for both states, and Warsaw was to be the new capital. The Union lasted 226 years until its total abolition in 1795.
Twenty five years passed before civil unrest brought about the great Assembly at Brzsc in Lithuania in 1595, at which the Union was formally announced to the Lithuanian people. There resulted in considerable violence. The Poles set about 'Polonizing' whatever they could in Lithuania, and the Union was never cordial. Indeed, as late as 1919, the Poles had the audacity to state to the western Allies that the Lithuanians were a "primitive people" and only still existed because of Polish assistance and practical help!; and that Poland had "never imposed upon any nation our language or religion"!
The "Commonwealth"'s political system (known alternately as the 'Noble's Democracy' or 'Golden Freedom) was characterized by strict checks upon monarchical power effectively making it a constitutional monarchy and federation. These checks were enacted by a dominant Polish legislature (Sejm) controlled by the Polish nobility (szlachta).  The two component states of the "Commonwealth" were formally equal, yet Poland made certain it would be the dominant partner in the union. "Formally, Poland and Lithuania were to be distinct, equal components of the federation.....but Poland, which retained possession of the Lithuanian lands it had now seized, had greater representation in the Diet and became the dominant partner."
Ultimately the Commonwealth entered a period of protracted political, military and economic decline. Its growing weakness and constant wars led to its partitioning among its more powerful neighbors, Austria, Prussia and Russia, during the late 18th century. Shortly before its utter demise, the Commonwealth (or more correctly Poland), inspired by the new United States of America, adopted massive political reforms and enacted a new Constitution on May 3, 1791, which has been described as the second oldest codified national constitution of modern history.
- Wickham-Steed, H., Professor Phillips, Walter Alison, and Hannay, David, A History of Austria-Hungary and Poland, reproduced from the 11th edition of the Encyclopaedia Britannica, London, 1914, pps:117-8.
- Morfill, W.R., M.A., Russia, London, 1891, p.47.
- Woodward, Professor E.L., and Butler, Rohan, Documents on British Foreign Policy 1919-1939, First Series, vol.iii, 1919, HMSO, London, 1949, p.352.
- "Poland." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. Retrieved 20 Feb. 2009
- Heritage: Interactive Atlas: Polish-Lithuanian Commonwealth. Retrieved March 19, 2006: At its apogee, the Polish-Lithuanian Commonwealth comprised some 400000 sqmi and a multi-ethnic population of 11 million. For population comparisons, see also those maps: , .
- Norman Davies, Europe: A History, Pimlico 1997, p. 554.
- Yale Richmond, From Da to Yes: Understanding the East Europeans, Intercultural Press, 1995, p. 51
- Maciej Janowski, Polish Liberal Thought, Central European University Press, 2001, ISBN 963-9241-18-0, Google Print: p3, p12
- Paul W. Schroeder, The Transformation of European Politics 1763–1848, Oxford University Press, 1996, ISBN 0-19-820654-2, Google print p84
- Aleksander Gella, Development of Class Structure in Eastern Europe: Poland and Her Southern Neighbors, SUNY Press, 1998, ISBN 0-88706-833-2, Google Print, p13
- Encyclopædia Britannica, 2006, 'Union of Lublin'.
- Martin Van Gelderen, Quentin Skinner, Republicanism: A Shared European Heritage, Cambridge University Press, 2002, ISBN 0-521-80756-5 Google Print: p54
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- John Markoff describes the advent of modern codified national constitutions as one of the milestones of democracy, and states that "The first European country to follow the U.S. example was Poland in 1791." John Markoff, Waves of Democracy, 1996, ISBN 0-8039-9019-7, p.121.