Following the outbreak of World War II, Taylor joined Army Intelligence as a Major on October 5, 1942, leading the American group at Bletchley Park that was responsible for analyzing information obtained from intercepted German communications using ULTRA encryption. He was promoted to lieutenant colonel in 1943 and visited England, where he helped negotiate the 1943 BRUSA Agreement. He was promoted to full colonel in 1944, and was assigned to the team of Robert H. Jackson, which helped work out the London Charter of the International Military Tribunal (IMT), the legal basis for the Nuremberg Trials.