As the Soviets were fighting the Germans all along a 1000 mile long front, they were demanding that the Allies in the west do something to open a second front against the Germans.  We’ll get to the Allies eventual response to this in a couple of episodes, but the western allies had the same problem invading Europe that the Germans had had when they were thinking about invading Britain.  That problem was the English Channel.  

In order to send an invasion fleet across the channel to land in France, the Allies had to have complete control of the air above the channel, to protect the invasion fleet from German planes.  So before the sea-borne invasion could take place, the Allies needed to defeat the Luftwaffe, or at least drive them back from their bases in France where they could attack an invasion fleet.  

So the Allies started building up their air forces in Great Britain. The Royal Air Force, which had done such a heroic job of defeating the Luftwaffe in the Battle of Britain, had continued to build up its fighter and bomber forces.  But early 1942, they were joined by the newly formed American group, the Eighth Air Force.  The Americans began to ship men, planes, bombs and ammo, and fuel to Great Britain.  By the end of the war, the Americans are going to have over 200 airbases in Great Britain, with over half a million men on those bases, and over 50,000 planes.