Relations between Britain and Germany deteriorated after the Munich Conference as Hitler continued his military expansion. Germany invaded Poland in September 1939 using lightning-fast "blitzkrieg" tactics to defeat Poland within a month. France adopted a policy of collaboration with Germany after its defeat in June 1940. Germany then targeted Britain but was unable to invade due to the English Channel and powerful British navy, instead launching an aerial assault known as the Battle of Britain. Though outnumbered, the British Royal Air Force held off the German Luftwaffe in a major victory.