Hours after the Attack on Pearl Harbor, Japanese forces from the Marianas launched an invasion of Guam on December 8, 1941. Chamorros from the Northern Marianas, then under Japanese rule for more than two decades, were brought to Guam to assist the Japanese administration. This fact, combined with the harsh treatment of Guamanian Chamorros during the brief 31-month occupation, created a rift between the two populations that would become the main reason Guamanians rejected reunification referendum approved by the Northern Marianas in the 1960s.