On February 27, 1531, John joined Philip I, Landgrave of Hesse, in establishing the Schmalkaldic League, a defensive military alliance of Lutheran principalities in which each pledged to support the other in the event of an attack by the forces of the Catholic Charles V, Holy Roman Emperor.
In time Anhalt, Württemberg, Pomerania, Augsburg, Frankfurt, Kempten, Brandenburg and the Electoral Palatinate were added to the League.
In view of the preparations of the Emperor Charles V to suppress them, the members of the League launched a pre-emptive attack on his forces at the Catholic city of Füssen on July 10, 1546.
Domestic pressure from the Protestant subjects of the empire led to the offer of additional concessions in the Leipzig Interim in December, and finally to more violence.
On January 15, 1552, a coalition of Protestant princes of the Holy Roman Empire signed the Treaty of Chambord with King Henry II of France, inviting him to occupy the Three Bishoprics of Metz, Verdun and Toul in exchange for military assistance against Charles.