Other states within the Holy Roman Empire subsequently joined the treaty, and their exit was a key factor in Catholic France entering the war as an ally of Protestant Sweden.

[citation needed] Christian IV of Denmark invaded Northern Germany in support of his fellow Protestants until forced to withdraw in 1629.

Success led Ferdinand to pass the Edict of Restitution, which required any property transferred since 1552 to be restored to its original owner, which was in nearly every case the Catholic Church.

To strengthen its borders in the Rhineland and Low Countries, France supported the Dutch against the Swedes in the Baltic, and also backed Maximilian of Bavaria, a leader of the anti-Swedish Catholic League.

[8] After 1632, Ferdinand accepted Catholicism could not be re-imposed by force and opened discussions on amending the Edict of Restitution in February 1633, eighteen months before Nördlingen.

With the Lutheran states of Denmark-Norway and Hesse-Darmstadt acting as mediators, the two parties agreed a preliminary draft in November 1634, known as the Pirnaer Noteln.

[13] While Ferdinand continued the Counter-Reformation in his own lands, it is generally agreed the Peace of Prague ended it as an internal religious conflict and re-established the principle of cuius regio, eius religio.

By renouncing their right to create alliances and handing over control of armed forces, the Imperial estates, in return, acknowledged the supremacy of the Emperor.