Field Marshal Sir John Greer Dill, GCB, CMG, DSO was a senior British Army officer with service in both the First World War and the Second World War. From May 1940 to December 1941 he was the Chief of the Imperial General Staff, the professional head of the British Army, and subsequently served in Washington, D.C., as Chief of the British Joint Staff Mission and then Senior British Representative on the Combined Chiefs of Staff.