9. Armee
During World War II he commanded the 9th Infantry Regiment and was the assistant division commander of the 2nd Infantry Division. In succession he commanded the 35th and the 30th Infantry Divisions, the XII Corps, and the Fourth Army. In May 1944, with the three-star rank of lieutenant general, he assumed command of the Ninth Army. Simpson led the Ninth Army in the assault on Brest in September 1944, and the advance to the Roer River in November. During the Battle of the Bulge in December, Simpson's Ninth Army came under command of Field Marshal Bernard Montgomery's Anglo-Canadian 21st Army Group. After the battle was over in early 1945, the Ninth Army remained with Montgomery's 21st Army Group for Operation Grenade, the advance to the Rhine, and Operation Plunder, its crossing. On April 1 the Ninth Army made contact with the First Army, making a complete encirclement of the Ruhr, and on April 11, it reached the Elbe.