A pilot in the Army Air Force in World War II, Lieutenant Guill flew combat missions over Germany, was shot down near Hamburg, and spent the last few months of the war in a German prison camp. He completed his undergraduate work after the war, taking a B.S. in chemistry from the University of California in 1947. He was awarded a master’s degree in history from the same institution in 1951 and commissioned in the naval reserve in 1952.