The First War of Kappel (Erster Kappelerkrieg) was an armed conflict in 1529 between the Protestant and the Catholic cantons of the Old Swiss Confederacy during the Reformation in Switzerland. It ended, without any single battle having been fought, with the first peace of Kappel (Erster Landfriede).
Under the lead of Huldrych Zwingli, the Protestant canton and city of Zürich had concluded with other Protestant cantons a defence alliance, the Christliches Burgrecht, which also included the free imperial cities of Konstanz and Strasbourg. The Catholic cantons in response had formed an alliance with Ferdinand of Austria.
Conflicts between the two sides arose also over the situation in the common territories, especially the Thurgau, where the administration changed bi-annually between cantons and which thus switched between Catholic and Protestant rules. Several mediation attempts failed, such as the disputation of Baden in 1526.
After numerous minor incidents and provocations from both sides, a Catholic priest was executed in the Thurgau in May 1528, and the Protestant pastor J. Keyser was burned at the stake in Schwyz in 1529. The last straw was the installation of a Catholic vogt at Baden. Zürich declared war on 8 June, occupied the Thurgau and the territories of the Abbey of St. Gall, and marched to Kappel at the border to Zug.
By mediation at the Federal Tagsatzung, open war was barely avoided. While the armies were on the field (the march of Kappel between Zürich and Zug) and negotiations were ongoing, the soldiers of the two armies arranged to avoid all mutual provocation. Johannes Salat of Lucerne, who was an eye-witness, records how the men from both camps fraternised, drinking and talking together. Heinrich Bullinger later cast this in terms of the Kappeler Milchsuppe or "milk soup of Kappel", an anecdotal account of how a meal was shared by the two armies, the side of Zurich providing the bread and the side of Zug the milk. This became a lasting symbol of reconciliation and compromise between confederates.
The peace agreement (Erster Landfriede) was not exactly favourable for the Catholic party, who had to dissolve its alliance with the Austrian Habsburgs. The tensions remained essentially unresolved, and would flare again in the Second War of Kappel two years later.