Complex sentence: A Department of External Affairs was created in 1919 but its functions were limited to administering New Zealand's Island Territories in the Pacific; namely the Cook Islands, Niue, Tokelau, and the League of Nations Mandate of Samoa.
Split into simpler sentences: A Department of External Affairs was created in 1919 but it did not deal with external relations. Instead, it was responsible for administrating New Zealand's Island Territories in the Pacific; namely the Cook Islands, Niue, Tokelau, and the League of Nations Mandate of Samoa.