“Paintings don’t change wars,” said the American painter Leon Golub in 1967. “They show feelings about wars.” And the feelings that exploded onto canvases and into sculpture during the Vietnam War changed art in America. Niceties were napalmed. Expression roared into new mediums. “Artists Respond,” which originated at the Smithsonian American Art Museum, presents approximately 100 volatile works by 58 artists. A must-see show. —L.J.