With the rank of captain, he was a company commander in the 11th Machine Gun Battalion, part of the 4th Division of the American Expeditionary Forces (AEF).

[2][5] In March 1942, three months after the Japanese attack on Pearl Harbor and the subsequent American entry into World War II, Eddy was promoted to the one-star general officer rank of brigadier general on March 24, 1942, and became the assistant division commander (ADC) of the 9th Infantry Division.

[9] After training in the United States for several months Eddy led the division overseas, landing in French North Africa on November 8, 1942, as part of Operation Torch.

The 9th Division fought in the subsequent Allied campaign in North Africa, and played a large role in the Battle of Kasserine Pass in February 1943.

For his role in the capture of the French port of Cherbourg,[10] which much impressed his superiors, Eddy was awarded the Distinguished Service Cross.

Eddy led XII Corps in the Allied advance from Paris to the Rhine and in the subsequent fighting on the Western Front.

In April 1945, after taking part in the Western Allied invasion of Germany, shortly before the end of World War II in Europe, Eddy returned to the United States due to a severe illness (extreme hypertension, from which he eventually recovered) and was replaced in command of XII Corps by Major General Stafford LeRoy Irwin.