The U.S. Navy's naval intelligence learned about the existence of the Japanese super-battleships IJN Yamato and IJN Musashi through a combination of radio intercepts, prisoner-of-war (POW) interrogations, aerial reconnaissance, and combat encounters. Here's a detailed timeline and explanation of how this occurred:
Pre-War Hints and Japanese Secrecy
Early Suspicions (Mid-1930s):
The Japanese began construction of the Yamato-class battleships in the late 1930s under extreme secrecy.
Their existence was rumored within naval intelligence circles due to Japan's known withdrawal from the Washington and London Naval Treaties, which had previously limited battleship size.
The U.S. suspected Japan might build ships exceeding the treaty limits, but the exact details remained unclear.
Construction Phase and Initial Intelligence (1937-1941)
Construction Observed by Spies:
The Yamato was constructed at Kure Naval Arsenal, and the Musashi at Nagasaki. Japan took extraordinary measures to conceal these ships, including constructing large scaffolding and screens to obscure their shape.
Local espionage reports hinted at the construction of a new class of battleships, but specifics like displacement, armament, and armor remained vague.
Radio Traffic Analysis:
U.S. cryptographers, who had broken parts of Japanese naval codes (e.g., JN-25), intercepted messages about new capital ships but couldn't confirm their specifics.
Combat Encounters and Increased Knowledge (1942-1944)
Battle of Midway (June 1942):
Intelligence analysts reviewing intercepted communications noted references to large, new battleships in the Japanese fleet. However, these ships did not participate in the battle, leaving their true nature still uncertain.
First Sighting by U.S. Pilots (1943):
In late 1943, U.S. reconnaissance aircraft spotted the Yamato and Musashi in the Truk Lagoon, a major Japanese naval base. Pilots reported seeing unusually large battleships, but their reports were initially met with skepticism due to the ships’ unprecedented size.
Prisoner Interrogations:
Captured Japanese sailors and officers provided fragmented information about massive battleships with extraordinary firepower and armor. These reports were gradually pieced together by U.S. intelligence.
Confirmation and Analysis (1944)
Battle of the Philippine Sea (June 1944):
The Yamato and Musashi were observed during this major naval battle. U.S. pilots flying reconnaissance and attack missions provided more detailed visual accounts, including estimates of size and armament.
Battle of Leyte Gulf (October 1944):
Both the Yamato and Musashi participated in this pivotal battle. The Musashi was sunk during the battle, giving the U.S. Navy an opportunity to study its wreckage and survivor accounts, which confirmed many of its specifications.