Kalsu was a starting guard in 1968. He played the entire season and was the Bills' team rookie-of-the-year. Following the 1968 season, to satisfy his Reserve Officers' Training Corps (ROTC) obligation, he entered the Army as a Second Lieutenant and arrived in Vietnam in November 1969 as part of the 101st Airborne Division. He was killed in action on July 21, 1970 when his unit came under enemy mortar fire at FSB Ripcord near the A Shau Valley. His family, out of respect, refused to talk in detail about the circumstances surrounding his death.
When he had left for Vietnam, Lieutenant Kalsu had to say goodbye to his wife, Jan, and his daughter Jill. On July 23, 1970, two days after his death, Jan Kalsu gave birth to his son, James Robert Kalsu Jr, at the Kalsu home in Oklahoma City. Mrs. Kalsu was informed of her husband's death only hours later. Kalsu and former Cleveland BrownDon Steinbrunner were the only professional football players to lose their lives in the Vietnam War.