By the winter of 1941/1942 both the British and German strategic bombing campaigns had reached a low ebb. The German offensive, a nine-month period of night bombing known as the Blitz, which had left London and many other British cities heavily damaged, had come to an end in May 1941, when the "Luftwaffe" had switched its resources to the invasion of the Soviet Union. Thereafter it had confined itself to hit-and-run raids on British coastal towns. Meanwhile, the RAF's night bombing offensive had been shown to be largely ineffective, as revealed by the Butt report in August 1941, and by Christmas the offensive had largely petered out.