By the mid-1930s the Japanese government abandoned the pretense of developing the Pacific islands for the benefit of the inhabitants and began building a series of fortifications in the region, planning for expansion in Southeast Asia. The substantial number of garrisons erected prolonged the conflicts during World War II (1939–1945), as the allied forces opted for a strategy of retaking island possessions rather than a direct strike on Japan. The Gilbert and Marshall Islands campaign began in 1943 and by early 1944, the Marshall Islands had been captured by US forces. At the end of the war, the United States proposed to retain control over the former Japanese-mandated islands for security purposes. In 1947, a trust agreement was drawn with the United Nations and the United States to establish the Trust Territory of the Pacific Islands.