Klare served as a navigator on a B-17 Flying Fortresss in the Eighth Air Force in England. On December 31, 1944, he was shot down over Germany. He spent the rest of World War II in German POW camps. He was liberated by advancing Soviet troops from Stalag Luft One on May 1, 1945. He was honorably discharged as a First Lieutenant in December, 1945, receiving the Air Medal, the Purple Heart, European area ribbons and the Prisoner of War medal.