Geiger commanded the III Amphibious Corps in the Battle of Okinawa, where he assumed command of the U.S. Tenth Army upon the combat death of Lt. General Simon Bolivar Buckner, Commanding General of the Tenth Army. Geiger led the Tenth Army until relieved by General Joseph Stilwell.
For his part in this action he was awarded the Distinguished Service Medal. His citation reads in part, "Going ashore with the early landing elements on April 1, 1945, he began a bitter three-month campaign ... with outstanding professional skill, forceful leadership and unswerving determination, he directed his units ... repeatedly disregarding personal safety to secure a first hand estimate of the battle situation and inspiring his men to heights of bravery and accomplishment."
Following attendance at the Marine Officers' School at Port Royal, South Carolina, he served as a member of the Marine detachments aboard Wisconsin (BB-9) and Delaware (BB-28). In August 1912, he was assigned to Nicaragua, where he participated in the bombardment, assault and capture of the hills called Coyotepe and Barranca. Further foreign shore duty followed in the Philippines and China with the First Brigade and with the Marine Detachment, American Legation, Peking, China, from 1913 to 1916.
In August 1927, he returned to Quantico as a squadron officer and instructor at the Marine Corps Schools, and in May 1928, was assigned to duty in the Aviation Section, Division of Operations and Training, at Marine Corps Headquarters. After attending the U.S. Army War College and graduating in June 1929, he was ordered to Quantico, where he was assigned duty as Commanding Officer, Aircraft Squadrons, East Coast Expeditionary Force. He returned to Washington for duty with Aeronautics, Navy Department as Officer in Charge, Marine Corps Aviation.
In June 1935, he returned to Quantico as Commanding Officer, Aircraft One, Fleet Marine Force. From June 1939 to March 1941, he was a student at the Senior and the Advanced Courses, Naval War College, Newport, Rhode Island. This was followed with a brief tour of duty in the Office of the Naval Attache, London.
He led the 1st Marine Aircraft Wing (known as the Cactus Air Force) from September 3, to November 4, 1942, while stationed at Guadalcanal. For extraordinary heroism in this capacity as well as commander of all aircraft, he was awarded a Gold Star in lieu of a second Navy Cross. His citation reads in part, "Despite almost continuous bombardment by enemy aircraft, hostile naval gunfire and shore based artillery, the combined total of Army, Navy and Marine Corps units stationed at Guadalcanal under Major General Geiger's efficiently coordinated command succeeded in shooting down 268 Japanese planes in aerial combat and inflicting damage on a number estimated to be as great…Sank six enemy vessels, including one heavy cruiser, possibly sank three destroyers and one heavy cruiser, and damaged 18 other ships, including one heavy cruiser and five light cruisers."
He was recalled to Marine Corps Headquarters in May 1943, to become Director of Aviation. In November 1943, he returned to the field, this time as Commanding General of the I Amphibious Corps and led the Corps from November 9, to December 15, 1943, in the Battle of Bougainville, for which he was awarded the Distinguished Service Medal.
Redesignated III Amphibious Corps in April 1944, he led this organization in the invasion and subsequent recapture of Guam during July and August 1944, and in the assault and capture of the southern Palau Islands in September and October of the same year. For those operations he was awarded two Gold Award stars in lieu of a second and third Distinguished Service Medal.
Geiger led this Corps into action for the fourth time as part of the Tenth Army in the invasion and capture of Okinawa. In July 1945, he assumed duties as Commanding General of the Fleet Marine Force, Pacific, which position he held until called back to Headquarters Marine Corps in November 1946.