Part of Hitler’s platform in his rise to power involved creating “enemies” of the Reich. One of those was communism. Hitler knew that an alliance with Stalin would undermine support for communism abroad ( which it in fact did ), but he also knew that such an alliance would put the lie to one of his primary pillars of Nazism… the destruction of communism. His need for peace ( however temporary ) on the Eastern Front to allow him to concentrate on Operation Sea Lion ( the proposed invasion of England ) overrode his distaste of communism… temporarily.