XII Corps became operational in France as part of Lieutenant General George S. Patton's Third Army on 1 August 1944.

The XII Army Corps was not ordered to active duty prior to World War II, and was located in Manhattan as of 7 December 1941 in a reserve status.

Assembling south of Le Mans on 13 August 1944, XII Corps began driving eastward and liberated the cities of Orléans and Châteaudun within five days.

At the beginning of September 1944, serious gasoline shortages forced an abrupt halt to General Patton's pursuit across northern France.

Resuming its advance on 4 September, but facing shortages of gasoline and ammunition because of the allied logistical crisis, XII Corps now confronted a vastly changed tactical situation.

XII Corps assaulted across the Moselle River at Dieulouard on 13 September 1944, and heavy fighting ensued for three days as the Germans attempted to collapse the bridgehead.

After resting and reorganizing for a month, XII Corps opened an offensive to reach the Saar River on 8 November 1944.

On 22 March 1945, XII Corps assaulted across the Rhine River at Oppenheim without involved preparation and caught the Germans in the area off-guard.

On 26 March 1945, the corps assaulted across the Main River into Frankfurt am Main, and fought for three days to capture the city in the face of German resistance characterized by the use of assault guns and a heavy concentration of Flak artillery in the dense urban environment.

On 4 April 1945 XII Corps troops discovered a Nazi stash of art treasures and gold in a salt mine at Merkers, and then took Meiningen the following day.

The so-called redoubt proved to be a myth and the commitment of eight U.S. and French army corps against it was a curious use of allied military resources while other allied armies in the north paused for weeks only 80 miles from the true heart of German resistance, Berlin.

Operating simultaneously in Germany and Czechoslovakia, the corps crossed into Austria and assaulted across the Danube River southwest of Regensburg on 26 April 1945.

XII Corps is credited with service in the Northern France, Rhineland, Ardennes-Alsace, and Central Europe campaigns.