Truman was an artillery officer in WW I. His unit was firing toward German lines at 11:00 am/11/11/1918.
In 1945, he orders the dropping of the atomic bomb to end WW II.
While unprovable, it is possible Harry Truman (in essence) fired the last shot of both World Wars.
Perhaps the last shot fired by an American, but a US airman was killed on 8/18/45 by a Japanese fighter pilot who cut loose on an American photo-recon mission over the home islands.