The Schmalkaldic League was a defensive league of Protestant princes in the Holy Roman Empire in the mid-16th century.
The League was assembled by Philipp I of Hesse and John Frederick, Elector of Saxony at Schmalkalden in 1531, pledging to defend each other if their territories were attacked by Charles V, Holy Roman Emperor. Anhalt, Bremen, Brunswick-Lüneburg, Magdeburg, Mansfeld , Strassburg, and Ulm were the other original members. Constance, Reutlingen, Memmingen, Lindau, Biberach an der Riß, Isny , Lübeck, and Bavaria later joined as well. The League agreed to provide 10,000 troops and 2000 knights for their mutual protection.
In 1532 the League allied with France, and in 1538 with Denmark. The League rarely provoked Charles directly, but confiscated Church land, expelled bishops and Catholic princes, and helped spread Lutheranism throughout northern Germany.
In 1544 Charles V made peace with France, with France agreeing to end their alliance. Charles and Pope Paul III began to gather an army in 1546, while the members of the League bickered amongst themselves, unable to unite in defense as they had originally planned. Charles defeated the League at the Battle of Mühlberg on April 24, 1547, capturing many of its leaders. However, new Protestant leagues were created, eventually leading to the Peace of Augsburg in 1555.