Complex sentence: The city of Hanover was not part of the district until 2001, when the district and the city were merged in order to form the new "Region Hannover" district (not to be confused with the administrative region of Hanover).
Split into simpler sentences: The city of Hanover was not part of the district until 2001. Then the district and the city were merged in order to form the new "Region Hannover" district (not to confuse with the administrative region of Hanover).