Polish-Lithuanian union

The term Polish–Lithuanian Union sometimes called the United Kingdom of Poland and Lithuania refers to a series of acts and alliances between the Kingdom of Poland and the Grand Duchy of Lithuania that lasted for prolonged periods of time and led to the creation of the Polish–Lithuanian Commonwealth—the "Republic of the Two Nations"—in 1569 and eventually to the creation of a short-lived unitary state in 1791.

Important events in the process of union included:

...
Wikipedia