The Cologne War was a conflict between Protestant and Catholic factions that devastated the Electorate of Cologne, a historical ecclesiastical principality of the Holy Roman Empire, within present-day North Rhine-Westphalia, in Germany. The war occurred within the context of the Protestant Reformation in Germany and the subsequent Counter-Reformation, and concurrently with the Dutch Revolt and the French Wars of Religion.
Also called the Seneschal's War or the Seneschal Upheaval and occasionally the Sewer War, the conflict tested the principle of ecclesiastical reservation, which had been included in the religious Peace of Augsburg. This principle excluded, or "reserved", the ecclesiastical territories of the Holy Roman Empire from the application of cuius regio, eius religio, or "whose rule, his religion", as the primary means of determining the religion of a territory. It stipulated instead that if an ecclesiastical prince converted to Protestantism, he would resign from his position rather than force the conversion of his subjects.
In December 1582, Gebhard Truchsess von Waldburg, the Prince-elector of Cologne, converted to Protestantism.