Operation Vengeance was the American military operation to kill Admiral Isoroku Yamamoto of the Imperial Japanese Navy on 18 April 1943 during the Solomon Islands campaign in the Pacific Theater of World War II. Yamamoto, commander of the Combined Fleet of the Imperial Japanese Navy, was killed near Bougainville Island when his G4M1 transport aircraft was shot down by United States Army Air Forces fighter aircraft operating from Kukum Field on Guadalcanal.
The mission of the U.S. aircraft was specifically to kill Yamamoto, made possible because of United States Navy intelligence decoding transmissions about Yamamoto's travel itinerary through the Solomon Islands area. The death of Yamamoto reportedly damaged the morale of Japanese naval personnel, raised the morale of the Allied forces, and was intended as revenge by U.S. leaders, who blamed Yamamoto for the attack on Pearl Harbor that initiated the war between Imperial Japan and the United States.
The U.S. pilots claimed to have shot down three twin-engine bombers and two fighters during the mission, but Japanese records show only two bombers were shot down. There is a controversy over which pilot shot down Yamamoto's plane, but most modern historians credit Rex T. Barber.
Admiral Isoroku Yamamoto, commander of the Imperial Japanese Navy, scheduled an inspection tour of the Solomon Islands and New Guinea. He planned to inspect Japanese air units participating in Operation I-Go that had begun 7 April 1943; in addition, the tour would boost Japanese morale following the disastrous Guadalcanal campaign and its subsequent evacuation during January and February.
On 14 April the U.S. naval intelligence effort code-named "Magic" intercepted and decrypted orders alerting affected Japanese units of the tour. The original message, NTF131755, addressed to the commanders of Base Unit No. 1, the 11th Air Flotilla, and the 26th Air Flotilla, was encoded in the Japanese Naval Cipher JN-25D, and was picked up by three stations of the "Magic" apparatus, including Fleet Radio Unit Pacific Fleet. The message was then deciphered by Navy cryptographers (among them future Supreme Court Justice John Paul Stevens); it contained time and location details of Yamamoto's itinerary, as well as the number and types of planes that would transport and accompany him on the journey.
The decrypted text revealed that on 18 April Yamamoto would fly from Rabaul to Balalae Airfield, on an island near Bougainville in the Solomon Islands. He and his staff would fly in two medium bombers (Mitsubishi G4M Bettys of the Kōkūtai 705), escorted by six navy fighters (Mitsubishi A6M Zero fighters of the Kōkūtai 204), to depart Rabaul at 06:00 and arrive at Balalae at 08:00, Tokyo time.
The U.S. Army Military Intelligence Service (MIS) was made up mostly of Nisei (second-generation Japanese Americans). They were trained in interpretation, interrogation, and translation of Japanese materials ranging from standard textbooks to captured documents. Information leading to Yamamoto's ambush was a significant MIS contribution in the Solomons campaign. MIS Technical Sergeant Harold Fudenna translated an intercepted radio message indicating the itinerary of Yamamoto. Although this message was first met with skepticism that the Japanese would be so careless, other MIS linguists in Alaska and Hawaii had also intercepted the same message, confirming its accuracy.
President Franklin D. Roosevelt may have authorized Secretary of the Navy Frank Knox to "get Yamamoto", but no official record of such an order exists, and sources disagree whether he did so. Knox essentially let Admiral Chester W. Nimitz make the decision. Nimitz first consulted Admiral William F. Halsey Jr., Commander, South Pacific, and then authorized the mission on 17 April. These U.S. commanders judged that the benefits of a successful mission included that Japanese morale would be negatively affected by news of Yamamoto's death and that Yamamoto's replacement would be less capable. When the issue was raised that the mission could reveal that the United States had broken Japanese naval codes, the commanders decided the knowledge could be protected as long as the true source of the intelligence was kept hidden from unauthorized American personnel and the press.
To avoid detection by radar and Japanese personnel stationed in the Solomon Islands along a straight-line distance of about 400 miles (640 km) between U.S. forces and Bougainville, the mission entailed an over-water flight south and west of the Solomons. This roundabout approach was plotted and measured to be about 600 miles (970 km). The fighters would, therefore, travel 600 miles out to the target and 400 miles back. The 1,000-mile flight, with extra fuel allotted for combat, was beyond the range of the F4F Wildcat and F4U Corsair fighters then available to Navy and Marine squadrons based on Guadalcanal. The mission was instead assigned to the 339th Fighter Squadron, 347th Fighter Group, whose P-38G Lightning aircraft, equipped with drop tanks, were the only American fighters in the Pacific with the range to intercept, engage and return.
339th Squadron Commander Major John W. Mitchell, already an ace pilot, was chosen to lead the flight. For better navigation, Mitchell asked for a navy compass, which was provided by Marine Corps Lieutenant Colonel Luther S. Moore and installed in Mitchell's P-38 the day before the attack. All of the P-38 fighters mounted their standard armament of one 20 mm cannon and four .50-caliber (12.7 mm) machine guns, and were equipped to carry two 165-US-gallon (620 L) drop tanks under their wings. A limited supply of 330-US-gallon (1,200 L) tanks was flown up from New Guinea, sufficient to provide each Lightning with one large tank to replace one of the small tanks. The difference in size put approximately 1,000 pounds (450 kg) greater weight on one side of the aircraft, but the tanks were located close enough to the aircraft's center of gravity to avoid serious performance problems.
Eighteen P-38s were assigned the mission. One flight of four was designated as the "killer" flight, while the remainder, which included two spares, would climb to 18,000 feet (5,500 m) to act as "top cover" for the expected reaction by Japanese fighters based at Kahili. A flight plan was prepared by the Command Operations Officer, Marine Major John Condon, but this was discarded by Mitchell who thought the airspeeds and time estimates were not best for intercepting Yamamoto. With several of his pilots assisting, Mitchell calculated an intercept time of 09:35, based on the itinerary, to catch the bombers descending over Bougainville, 10 minutes before landing at Balalae. He worked back from that time and drew four precisely calculated legs, with a fifth leg curving in a search pattern in case Yamamoto was not seen at the chosen point. In addition to heading out over the Coral Sea, the 339th would "wave-hop" all the way to Bougainville at altitudes no greater than 50 feet (15 m), maintaining radio silence.
Although the 339th Fighter Squadron led the mission, 10 of the 18 pilots were drawn from the other two squadrons of the 347th Group, (8 from the 12th FS and 2 from the 70th FS). The Commander AirSols, Rear Admiral Marc A. Mitscher, selected four pilots to be designated as the "killer" flight:
Captain Thomas G. Lanphier Jr.
Lieutenant Jim McLanahan (dropped out with flat tire)
Lieutenant Joe Moore (dropped out with faulty fuel feed)
The remaining pilots would act as reserves and provide air cover against any retaliatory attacks by local Japanese fighters:
A briefing included the designated cover story for the source of the intelligence stating it had come from Australian coastwatchers, who supposedly had spotted an important high-ranking officer boarding an aircraft at Rabaul. Several historians say that the pilots were not specifically briefed on the identity of their target, but Thomas Alexander Hughes writes that Mitscher told the assembled pilots it was Yamamoto, to "provide additional incentive" to the fliers.