As a WWI veteran, Hamlin felt compelled to participate in the second World War as well. He tried to enlist, but was turned down by the recruiting officer who told him that 42 was too old to be sent to fight. Instead, he supported the United States by visiting sick soldiers and using his artistic skills and comic strip storylines to promote the military. He would often hand out his original artwork for his strips to the soldiers he met in the hospitals. While many contemporary comic strip characters fought in the war, Alley Oop wasn't able to, but like Hamlin he brought his unique abilities to the war effort.
The 92nd Bomb Group adopted Alley Oop as their mascot, and Hamlin assisted them, painting Oop on their pilot's jackets and the noses of their B-17s. They became known as the "Oop Group," and participated in the bombing of Nazi targets in Germany and Austria.
Alley Oop tried to enlist but was refused because he wasn't a U.S. citizen, mirroring Hamlin's frustration at not being able to fight. However, he still helped out in whatever way he could, whether it was turning invisible and sneaking on a US Navy ship bound for the Pacific, enlisting the aid of dinosaurs from Moo in the effort, or advertising war bonds on the side of Dinny.
In addition to visiting wounded servicemen, Hamlin drew a series of cartoons for an Air Force Hospital in Nashville, depicting the steps a wounded serviceman would have to take during the convalescence process. They were published in Review, the hospital's magazine, in September 1945.