Complex sentence: There is ample documentary evidence that Hitler desired to eradicate Jewry, and that the order to do this when he attained power did indeed originate from him, including the authorization of mass deportations of the Jews to the east beginning in October 1941.
Split into simpler sentences: There is ample documentary evidence that Hitler desired to eradicate Jewry, and that the order to do this when he attained power did indeed originate from him. Including authorized the mass deportations of the Jews to the east beginning in October 1941.