Whiteley joined Lieutenant General Dwight D. Eisenhower's Allied Forces Headquarters as the British Deputy Chief of Staff, working with the American Deputy Chief of Staff, Brigadier General Alfred Gruenther, and the Chief of Staff, Brigadier General Walter Bedell Smith. As such, Whiteley, who was duly promoted to major general in February 1943, was involved in the planning and direction of the Tunisian campaign and the Allied invasion of Sicily. In August 1943 he acted as an envoy once more, travelling to London to brief Churchill on plans for the Allied invasion of Italy.