[3] After the American entry into World War I (in April 1917) Walker was sent overseas and served with the 30th Infantry Regiment, part of the 3rd Division, into the trenches of the Western Front.

Those who succeeded in crossing were thrown into such confusion that they were unable to follow the barrage; and, through the effective leadership of this officer, no Germans remained in his sector south of the river at the end of the day's action.

After attending and later graduating from the United States Army War College in 1933, he remained there as an instructor for well over three years, where he taught many students who would later serve with in the not too distant future, such as Mark W. Clark, Courtney Hodges, Louis E. Hibbs and Omar Bradley.

The assault landings were successful, although the division suffered heavy casualties when the German troops launched numerous counterattacks in an attempt to push the Allies back into the sea.

[7] Walker commanded the 36th Division during the whole of its service in the grueling slog up Italy, crossing the Volturno Line in October, and by late November/early December, by which time the division was part of Major General Geoffrey Keyes' U.S. II Corps, were fighting in front of the Bernhardt Line, part of the formidable Winter Line defenses, suffering heavy casualties in the Battle of San Pietro Infine, later participating in the Rapido river crossing, part of the First Battle of Monte Cassino, and Monte Artemisio on the drive north through Rome and beyond.

The fighting in the early months of the Allied campaign in Italy proved very costly for the 36th Division, due to determined German resistance, the mountainous terrain, and the worsening winter weather.

"[8] Unfortunately, the crossing of the Rapido that took place on January 20–22, 1944 was a total failure, which resulted in heavy losses for the 36th Division, suffering approximately 1,681 casualties–143 killed, 663 wounded and 875 missing.

After the war, the Thirty-Sixth Division Veteran Association called for a Congressional investigation of this battle, due to the inefficiency and inexperience of General Clark.

"[10] In July 1944, Walker was transferred back to the United States and appointed as Commander of the U.S. Army Infantry School at Fort Benning, Georgia.