From November 26, 1943 to April 9, 1945, Seitz commanded the 26th Infantry Regiment (organized for the 1944–1945 campaign in France, Belgium and Germany as Regimental Combat Team 26 or Combat Team (CT) 26). CT 26 was a unit of the 1st Infantry Division (United States). Seitz commanded the combat team at Omaha Beach during the Normandy landings on D-Day, in the breakout from Normandy and at the approach to close the gap in the Falaise Pocket. The First United States Army, including Seitz's regimental combat team, hotly pursued the fleeing German Army divisions across France to the border of Germany by September 7, 1944. On October 11, 1944, after heavy fighting during the Battle of Aachen, Seitz led two of the three battalions of the regiment into heavily defended Aachen, Germany. They seized the first German city to fall to the Allies of World War II after 8 days of urban combat. In November 1944, Seitz led the regiment during the opening phase of the Battle of Hürtgen Forest. Starting only 10 days after the regiment was withdrawn from this bloody and costly battle, Seitz directed CT 26's heroic, successful stand at the , an important part of the Battle of the Bulge that followed the Battle of Elsenborn Ridge. With many replacement soldiers due to losses in the Hürtgen Forest, and with skillful artillery support, Seitz and the outnumbered 26th Infantry Regiment team prevented the Nazi 12th SS Panzer Division Hitlerjugend, 3rd Parachute Division (Germany) and the 12th Volksgrenadier Division, which were supported by dozens of tanks, from breaking the Allied line at Bütgenbach.