The Second War of Kappel () was an armed conflict in 1531 between the Catholic and the Protestant cantons of the Old Swiss Confederacy during the Reformation in Switzerland.
Background
thumb|250px|left|The [[Tagsatzung of 1531 in Baden failed to mediate between the parties (1790s drawing)]]
The peace concluded after the First War of Kappel two years earlier had prevented an armed confrontation, but the tensions between the two parties had not been resolved, and provocations from both sides continued, fuelled in particular by the Augsburg Confession of 1530. The Protestant canton of Zürich and Huldrych Zwingli, leader of the Swiss Reformation, feared a military action by Ferdinand I, Archduke of Austria and his brother Charles V, Holy Roman Emperor against Swiss Protestants, and saw the five Catholic cantons of Central Switzerland (Lucerne, Schwyz, Uri, Zug and Unterwalden) as potential allies of the two Habsburg sovereigns. Additionally, the Catholic party accused Zürich of territorial ambitions. While the Federal Diet (Tagsatzung) had successfully mediated in 1529, on this occasion the attempt failed, not least because Zwingli was eager to implement the Reformation throughout the Confederacy. After the measure failed to pressure the Catholics into concessions, in September Bern suggested lifting the embargo, which caused tensions with Zürich.
The renewed defeat led to increasing desertions among the Protestant army, which retreated down the Reuss valley to Bremgarten on 3 November.
In anticipation of the Cuius regio eius religio principle of the 1555 Peace of Augsburg, the Second Peace of Kappel confirmed each canton's right to determine the denomination of its own citizens and subjects, but favored Catholicism in the Confederacy's common territories.