A shift in Britain's leadership.

In April 1940 Hitler invaded Denmark and Norway. The latter would provide him naval bases so, he had been really determined to get it. What is more, he was confident that he would not be stopped by the British navy this time, as had happened in WWI. Eventually, the British did attempt to help Norway but this was a failure. Consequently, Prime Minister Chamberlain resigned and was replaced by Winston Churchill, who had already been First Lord of the Admiralty during WWI.