In a field outside Warsaw on 10 May 1573, Henri, duke of Anjou, was elected king of Poland. His elder brother, King Charles IX of France, appreciated that a Valois-ruled Poland, bordering on Austria, could prove very useful. As for the szlachta, so confident were they in their constitutional defences that even Protestants were prepared to accept a ruler widely deemed responsible for the previous year's St Bartholomew's massacre.

At the ‘Convocation’ Sejm in January 1573, the gentry's leaders had blustered the senatorial elite into conceding that all nobles were entitled to vote for their king viritim – in person. The Sejm formed an association, the ‘Confederacy of Warsaw’, which drew up constitutional ground rules. The king would not secure a successor vivente rege, during his own lifetime; he would have to preserve interdenominational peace; he would decide matters of peace and war in conjunction with the Senate and the Sejm; parliament was to be called every two years for a six-week term, or as necessary; its consent would be required for all extraordinary taxation. The nobility's jurisdiction over their peasants would remain untouched. If a ruler failed to observe his sworn commitments, he would forfeit his subjects' obedience. These ‘Henrician articles’ were complemented by the pacta conventa, obligations crafted for individual kings. King Henri's ranged from the provision of scholarships at the Sorbonne for young nobles to extravagant promises of financial and military support for Poland-Lithuania in the Muscovite war.