In 1938, Captain Merrill was assigned as assistant military attaché at the U.S. Embassy in Tokyo. Fascinated by East Asian culture, he studied both the history and languages of Japan and China. In early 1941, Major Merrill was transferred to Manila as an intelligence officer on the staff of General Douglas MacArthur. He was in Burma on a mission that December when the Japanese attacked the Philippines. He was then reassigned to the China-Burma-India Theater.
In spring 1942, Merrill was promoted to lieutenant colonel, but he was wounded in action in May when the Japanese pushed Allied forces out of Burma. In October 1943, Merrill became operations chief for Lieutenant General Joseph Stilwell and organized the 5307th Composite Unit (Provisional) in India, modeled after British Brigadier General Orde C. Wingate's Chindits. This regiment-sized infantry formation of nearly 3,000 men was designed for long-range penetration operations and relied entirely on air drops for supplies; it was the first U.S. ground unit to fight on the Asian mainland since the Boxer Uprising of 1900. Merrill assumed command of the 5307th in January 1944 and led Merrill's Marauders, as the outfit became known. Named Galahad, the 5307th took part in operations in north Burma during late February 1944, first in conjunction with two Chinese divisions and later with British Chindits. Over the next four months, the Marauders marched more than 1,000 miles through some of the densest jungles and over some of the highest peaks in Asia. Although consistently outnumbered, they participated in 5 major battles—Walabum, Shaduzup, Inkangahtawug, Nhpum Ga, and Myitkyina Airfield—and 30 minor engagements against the veteran Japanese 18th Division. The 5307th sustained significant losses due both to the fighting and to disease. By August, the unit was at one-third strength, and Stilwell decided to disband it.
Merrill suffered a heart attack in the middle of Galahad's actions and was replaced. Promoted to major general in September 1944, he became chief of staff of Tenth Army and fought in the conquest of Okinawa in 1945. In March 1947, Merrill was part of the U.S. military advisory group sent to the Philippines as it gained full independence. He retired from the army in 1948 and then became highway commissioner of the state of New Hampshire. Merrill died on 12 December 1955 at Fernandina Beach, Florida.
Bjorge, Gary J. Merrill's Marauders: Combined Operations in Northern Burma in 1944. Leavenworth, KS: Combat Studies Institute, U.S. Army Command and General Staff College, 1996.; Hunter, Charles N. Galahad. San Antonio, TX: Naylor, 1963.; Ogburn, Charlton. The Marauders. New York: Harper, 1959.