<P> Numerous bombing runs were launched by the United States aimed at the industrial heart of Germany . Using the high altitude B - 17, it was necessary for the raids to be conducted in daylight for the drops to be accurate . As adequate fighter escort was rarely available, the bombers would fly in tight, box formations, allowing each bomber to provide overlapping machine - gun fire for defense . The tight formations made it impossible to evade fire from Luftwaffe fighters, however, and American bomber crew losses were high . One such example was the Schweinfurt - Regensburg mission, which resulted in staggering losses of men and equipment . The introduction of the revered P - 51 Mustang, which had enough fuel to make a round trip to Germany's heartland, helped to reduce losses later in the war . In mid-1942, the United States Army Air Forces (USAAF) arrived in the UK and carried out a few raids across the English Channel . The USAAF Eighth Air Force's B - 17 bombers were called the "Flying Fortresses" because of their heavy defensive armament of ten to twelve machine guns, and armor plating in vital locations . In part because of their heavier armament and armor, they carried smaller bomb loads than British bombers . With all of this, the USAAF's commanders in Washington, DC, and in Great Britain adopted the strategy of taking on the Luftwaffe head on, in larger and larger air raids by mutually defending bombers, flying over Germany, Austria, and France at high altitudes during the daytime . Also, both the U.S. Government and its Army Air Forces commanders were reluctant to bomb enemy cities and towns indiscriminately . They claimed that by using the B - 17 and the Norden bombsight, the USAAF should be able to carry out "precision bombing" on locations vital to the German war machine: factories, naval bases, shipyards, railroad yards, railroad junctions, power plants, steel mills, airfields, etc . </P> <P> In January 1943, at the Casablanca Conference, it was agreed RAF Bomber Command operations against Germany would be reinforced by the USAAF in a Combined Operations Offensive plan called Operation Pointblank . Chief of the British Air Staff MRAF Sir Charles Portal was put in charge of the "strategic direction" of both British and American bomber operations . The text of the Casablanca directive read: "Your primary object will be the progressive destruction and dislocation of the German military, industrial and economic system and the undermining of the morale of the German people to a point where their capacity for armed resistance is fatally weakened .", At the beginning of the combined strategic bombing offensive on 4 March 1943 669 RAF and 303 USAAF heavy bombers were available . </P> <P> In the late 1943, the' Pointblank' attacks manifested themselves in the infamous Schweinfurt raids (first and second). Formations of unescorted bombers were no match for German fighters, which inflicted a deadly toll . In despair, the Eighth halted air operations over Germany until a long - range fighter could be found in 1944; it proved to be the P - 51 Mustang, which had the range to fly to Berlin and back . </P> <P> USAAF leaders firmly held to the claim of "precision bombing" of military targets for much of the war, and dismissed claims they were simply bombing cities . However the American Eighth Air Force received the first H2X radar sets in December 1943 . Within two weeks of the arrival of these first six sets, the Eighth command gave permission for them to area bomb a city using H2X and would continue to authorize, on average, about one such attack a week until the end of the war in Europe . </P>

When did the united states enter the second world war