Operation Undertone was a large assault by the U.S. Seventh and French 1st Armies of the U.S. Sixth Army Group as part of the Allied invasion of Germany in March 1945 during World War II. A force of three corps was to attack abreast from Saarbrücken, Germany, along a 75 km sector to a point southeast of Hagenau, France. A narrow strip along the Rhine leading to the extreme northeastern corner of Alsace at Lauterbourg was to be cleared by a division of the French 1st Army under operational control of the Seventh Army. The Seventh Army′s main effort was to be made in the center up the Kaiserslautern corridor.