Masters then returned to Pearl Harbor to join the 10th Defense Battalion in November 1942. Later, as executive officer of the battalion, attached to the I Marine Amphibious Corps, he served in the Solomon and Russell Islands. He was promoted to lieutenant colonel in April 1943. In August of that year, he commanded 2nd Battalion 1st Marines briefly in Australia prior to bringing the unit into combat again from December 1943 to February 1944 at the Battle of Cape Gloucester, again under the command of Rupertus, now the commanding general of the 1st Marine Division.