[4][5] Following the war, Parker commanded XXIII Corps during the occupation duties in Germany and completed his career as U.S. Army Provost Marshal General from 1948 to 1953.

On June 26, 1941, Parker was promoted to the temporary rank of colonel[5] and appointed commandant of the Field Artillery Replacement Training Center at Fort Bragg.

He was again promoted just a few months later, this time to the temporary rank of brigadier general on October 31, 1941, and was responsible for the training of artillery men until June 1942.

On June 21, 1942, Parker was subsequently promoted to the temporary rank of major general[5] and, despite being an artilleryman, was ordered to Camp Butner, North Carolina, where he was tasked with the activation of the 78th Infantry Division.

In March 1943, Parker's division began with its own intensive combat training and he led his command during the maneuvers in South Carolina during November 1943 and Tennessee in January-February 1944.

It was stationed near Bournemouth for four weeks and was ordered to the Tongres area, Belgium by the end of November as the part of reserve force of Ninth Army.

Parker then received orders to advanced into Simmerath, Witzerath, and Bikerath and attacked the village of Kesternich on the Siegfried Line.

Parker and his division then advanced to the Rhine river and participated in the capture of the Ludendorff Bridge during the Battle of Remagen by the beginning of March 1945.

Following the surrender of Germany, 78th Division conducted occupation duty in the areas of Kassel, Gotha, Fulda, Bad Hersfeld, Mühlhausen, Nordhausen and Marburg and Parker was tasked with the control of German civilian population or repatriation of Prisoners of War and displaced persons, people who had been brought into Germany by Nazis from conquered countries to be used as slave labor.

Following the deactivation of the Corps on February 10, 1946, Parker served as acting Commanding general, Third Army for several months until he was appointed Inspector-General, United States Forces European Theater by the end of August 1946.

Parker served as Deputy to general Walton Walker until April 1948, when he was appointed Provost Marshal of the United States Army.