His principal tasks were overseeing preparation of detailed plans and the buildup of men and materiel for Operation Overlord and supporting the Combined Bomber Offensive. At the end of May 1943, there were 133,000 US troops in the United Kingdom, of which 19,000 were in the ground forces, 74,000 in the air forces, and 34,000 in the services of supply. By the end of the year, their numbers had grown to 774,000, of which 265,000 were in the ground forces, 286,000 in the air forces, and 220,000 in the services of supply. In preparing the plans for Overlord, he worked closely with the Chief of Staff to the Supreme Allied Commander (COSSAC), British Lieutenant General Frederick E. Morgan with whom he had a good working relationship. Devers' main objection to the COSSAC plan was that he did not want American units smaller than corps directly subordinated to British command, based on what he called "the Pershing Principle." Devers clashed with Eisenhower over the diversion of ETOUSA resources to Eisenhower's North African Theater of Operations. On 28 July 1943, Eisenhower and Lieutenant General Carl Spaatz asked that four groups of Boeing B-17 Flying Fortress bombers be sent to the North African Theater to help support Operation Avalanche, the Allied invasion of Italy. The planned assault area at Salerno was at the extreme range of Allied fighters based in Sicily, and long-range bombers were needed to isolate the battlefield. Eisenhower and Spaatz felt that their theater should have priority. However, the four groups represented about a third of the Eighth Air Force's heavy bombers, and their departure would greatly impact the Combined Bomber Offensive, so Devers turned down the request. Devers and Eighth Air Force's commander, Brigadier General Ira C. Eaker, spoke to the British Chiefs of Staff Committee, who agreed with them. When Eisenhower appealed to Washington, Marshall and General Henry H. Arnold, the Chief of Army Air Forces, also supported Devers. In August, Eisenhower asked for permission to retain three groups of Consolidated B-24 Liberator bombers in the North African Theater. Devers again turned down his request, and Marshall and Arnold again supported him. In September, however, when Avalanche came under severe pressure from German counterattacks, Devers readily acceded to a request for the return of the three groups. Stephen Ambrose later noted, "Eisenhower was not accustomed to having his requests to Marshall turned down and found it difficult to