He sailed back to the Pacific theater in September 1944 and was appointed commanding officer of 3rd Battalion, 27th Marine Regiment under Colonel Thomas A. Wornham. Robertson was also promoted to the rank of lieutenant colonel in October 1944. During the Battle of Iwo Jima in February 1945, Robertson led his battalion up to the heavily fortified Hill 362, but his unit was pinned down by intense hostile mortar and machine-gun fire. He left his command post and moved to the forward observation post to personally observe the situation. Robertson then moved along the front line units, inspired his men to heroic effort in resuming the attack until they had advanced up the southern slopes and seized the crest of this vitally strategic hill. His battalion took heavy casualties, but he refused withdrawal of his unit and repulsed several night attacks and prevented the Japanese to penetrate the regimental line.