Barker enlisted as a Marine while working on Flying Tigers (1942). He was still filming his part as his reporting date approached, but the crew worked overtime to finish shooting his scenes. Shooting wrapped at 12:30 a.m., and he was with the Marines in San Diego seven-and-a-half hours later.
On February 25, 1945, he earned a Silver Star leading a platoon of the 5th Tank Battalion, 5th Marine Division on Iwo Jima.