In preparation for Operation Barbarossa, the invasion of the Soviet Union in June 1941, most fighter units were transferred to Germany's eastern borders by May 1941 but JG 2 and JG 26 were left in north-west Europe where their aim was interdiction of Royal Air Force (RAF) daytime operations over Europe. The two wings maintained around 120 serviceable aircraft against the increasing number of RAF Fighter Command sweeps conducted to wear down the Luftwaffe in a war of attrition. One of the busiest days of combat on the Channel coast in 1941 took place on 21 June, with two RAF Circus offensives flown.