During World War II, Rostow served in the Office of Strategic Services under William Joseph Donovan. Among other tasks, he participated in selecting targets for US bombardment. Nicholas Katzenbach later joked: "I finally understand the difference between Walt and me [...] I was the navigator who was shot down and spent two years in a German prison camp, and Walt was the guy picking my targets." In September 1942, Rostow arrived in London to serve as an intelligence analyst with the Enemy Objectives Unit, serving until the spring of 1945.