After a few months of medical leave, Williams returned to China in January 1943 and was assigned to the American embassy in Chungking as naval observer. He also simultaneously served with the US Naval Group China as instructor for Kuomintang guerrillas. He returned to the United States in July 1944 and after promotion to colonel, relieved Colonel James P. Riseley as commanding officer of the 6th Marine Regiment just after the Battle of Saipan. The 6th Marines sustained heavy casualties on Saipan and Williams supervised training and refitting of the regiment until the spring of 1945.