Dennis enlisted in the U.S. Army in 1967. He Graduated Army Airborne training at Fort Benning, Georgia, and later completed Special Forces Medical Training before going overseas. He volunteered for Vietnam and began serving there in April, 1969. His first assignment in Vietnam was Head Co. Field Medic with the First Cavalry Division and subsequently volunteered for Medevac. While serving in Vietnam, he was awarded the Silver Star, The Bronze Star with V-device (valor), two Air Medals (one with V-device) and the Combat Medic Badge.