information:classification, linguistic:paraphrasing
Union of Lublin (1569)

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.

Sigismund's death in 1572 was followed by an interregnum during which adjustments were made to the constitutional system; these adjustments significantly increased the power of the Polish nobility and established a truly elective monarchy.