The 1st Infantry Division (or Big Red One) saw its first combat in World War II in North Africa, landing at Oran and taking part in the initial fighting, 8-10 November 1942. Elements then took part in seesaw combat at Maktar, Medjez el Bab, Kasserine Pass, Gafsa, El Guettar, Beja, and Mateur, 21 January-9 May 1943, helping secure Tunisia.

The First was the first ashore in the invasion of Sicily, 10 July 1943 ; it fought a series of short, fierce battles on the island's tortuous terrain. When that campaign was over, the Division returned to England to prepare for the Normandy invasion.

The First Division assaulted Omaha Beach on D-day, 6 June 1944, some units suffering 30 percent casualties in the first hour, and secured Formigny and Caumont in the beachhead. The Division followed up the St. Lo break-through with an attack on Marigny, 27 July 1944, and then drove across France in a continuous offensive, reaching the German border at Aachen in September. The Division laid siege to Aachen, taking the city after a direct assault, 21 October 1944. The First then attacked east of Aachen through Hurtgen Forest, driving to the Roer, and moved to a rest area 7 December for its first real rest in 6 months' combat, when the von Rundstedt offensive suddenly broke loose, 16 December.

The Division raced to the Ardennes, and fighting continuously from 17 December 1944 to 28 January 1945, helped blunt and turn back the German offensive. Thereupon, the Division attacked and again breached the Siegfried Line, fought across the Roer, 23 February 1945, and drove on to the Rhine, crossing at the Remagen bridgehead, 15-16 March 1945. The Division broke out of the bridgehead, took part in the encirclement of the Ruhr Pocket, captured Paderborn, pushed through the Harz Mountains, and was in Czechoslovakia, at Kinsperk, Sangerberg, and Mnichov, when the war in Europe ended.