Churchill's greatest wartime fear, even after Hitler had suspended Operation Sea Lion and invaded the Soviet Union, was that the German submarine wolfpacks would succeed in strangling sea-locked Britain. A major factor that averted Britain's defeat in the Battle of the Atlantic was her regained mastery of Naval Enigma decryption; and while the latter benefited crucially from British seizure of German Enigma-equipped naval vessels, the breaking of German naval signals ultimately relied on techniques that had been pioneered by the Polish Cipher Bureau. Had Britain capitulated to Hitler, the United States would have been deprived of an essential forward base for its subsequent involvement in the European and North African theaters.