The king had arranged, before his death, for a strengthening of the alliance and personal union between the two countries via the Union of Lublin which merged the separate Polish and Lithuanian Parliaments and which provided for his own succession by an electoral monarchy. Henceforward, the king of the Commonwealth of Poland-Lithuania would be freely elected by the nobility of both nations.
On 16 May 1573, Henry Valois, third son of King Henri II of France and of Catherine de’Medici, Duke of Angoulême, Orléans, and Anjou was elected King of Poland and Grand Duke of Lithuania.
Several months prior to his election, on January 28, 1573, the Polish Parliament in Warsaw went out of parliamentary session and into the mode of a Confederacy (in order to preclude the use of the Liberum Veto.
The Confederacy of Warsaw pledged the entire nobility of Poland-Lithuania not only to refuse to enforce any measure by state or church undertaking to compel religious conformity, but to resist actively any such measure by armed force.
[W]e swear to each other, on behalf of ourselves and our descendants, in perpetuity, under oath and pledging our faith, honor, and consciences, that we who differ in matters of religion will keep the peace among ourselves, and neither shed blood on account of differences of Faith, nor punish one another by confiscation of goods, deprivation of honor, imprisonment, or exile.
During the debate, Crown Chancellor Jan Zamoyski is reported to have said: “For the heretics [Protestants] to return to the True Faith, I would give half my life’s blood, but to defend their right to obey their own consciences, I would give all my life’s blood.”