Major General Carl A. Spaatz took command of the Eighth Air Force in London in 1942; with Brigadier General Ira Eaker as second in command, he supervised the strategic bombing campaign. In late 1943, Spaatz was made commander of the new U.S. Strategic Air Forces, reporting directly to the Combined Chiefs of Staff. Spaatz began daylight bombing operations using the prewar doctrine of flying bombers in close formations, relying on their combined defensive firepower for protection from attacking enemy aircraft rather than supporting fighter escorts. The doctrine proved flawed when deep-penetration missions beyond the range of escort fighters were attempted, because German fighter planes overwhelmed U.S. formations, shooting down bombers in excess of "acceptable" loss rates, especially in combination with the vast number of flak anti-aircraft batteries defending Germany's major targets. American fliers took heavy casualties during raids on the oil refineries of Ploiești, Romania, and the ball-bearing factories at Schweinfurt and Regensburg, Germany, and it was the loss rate in crews and not materiel that brought about a pullback from the strategic offensive in the autumn of 1943.