On July 4, 1942, Ainsworth took administrative command of all Pacific Fleet destroyers (ComDesPac). On December 10 of that year, Admiral William F. Halsey gave him the additional duty of commanding Task Force 67 (TF 67) which had been badly mauled in the recent Battle of Tassafaronga; and, under his leadership that cruiser-destroyer force was soon winning renown as the "Ainsworth Express" for its fierce fighting in support of the final American drive to push Japanese troops off Guadalcanal. Its bombardment of the new Japanese air base at Munda on the island of New Georgia would be, in the words of naval historian, Samuel Eliot Morison, "... long regarded as a model..."