Robert T. Isham enlisted in the Army Enlisted Reserve in 1942, completing his college studies before active duty began in 1943. After basic training, he joined the Field Artillery, serving with the 18th Field Artillery Group and later the 693rd Field Artillery Battalion. By October, he was promoted to Corporal and soon after deployed to England for further training, then to France following D-Day.

Isham, fluent in French, played a key role as a forward observer during General Patton's advance across France, earning the Bronze Star for bravery under fire. His unit later supported the 3rd and 45th Infantry Divisions, where Isham maintained crucial communications under fire. After witnessing the devastation of war in Germany, including the Dachau death camp, Isham was struck by the somber reality of victory.

In early May 1945, Isham witnessed German forces surrender and visited Berchtesgaden, sending home wartime mementos. Later, he served near Salzburg and in Berlin under civilian assistant Laird Bell. Discharged in November 1945, Isham reflected on his wartime experiences as challenging yet profoundly meaningful, leaving him with a sense of fulfillment.