In April 1940 the war in Europe took an ominous turn. On June 22, 1940, France surrendered, and Britain and its Dominions stood alone against a formidable enemy. Although Britain managed to stave off Hitler's planned invasion, it remained under siege. London and the Channel ports were bombed unmercifully, and the U-boats, using their brand new bases in France, attacked convoys and independently-routed ships almost at will.