Cooper served with the 3rd Armored Division during World War II, and saw action from the Invasion of Normandy in 1944 through the Allied invasion of Germany in 1945. He was tasked with the "recovery, repair, and maintenance" of US tanks during the war. As part of his duties he regularly traveled through "the void", an area ranging from a few miles to as many as between the front lines and US supply trains, to deliver loss reports to commanders, which were too sensitive to transmit via radio. He refers to this as "running the gauntlet", due to the tendency of Allied armored forces to bypass some German elements in their advance, leaving pockets of enemy forces between the quickly advancing armor, and the trailing infantry units.