A court-martial is a military, airforce, or naval court which is enacted to decide on the guilt, or otherwise, of members of the army, airforce or navy. In the case of Captain Charles B. McVay III, the court-martial was held to establish whether McVay committed errors, or acted negligently, in his duty as captain aboard the USS Indianapolis.

The unit known as SIGINT, short for Signals Intelligence, was responsible for intercepting and interpreting Japanese radio transmissions and fleet signals. SIGINT had broken the Japanese communications code well before the Indianapolis set sail from Guam to Leyte, meaning that this group knew of the operation of the Tamon group of Japanese submarines, including the i-58, which had been dispatched to patrol the Marianas area west of Guam.

The top-secret information gathered by SIGINT (see above) was classified under the code word ULTRA, and in order to disguise the fact that the Japanese code had been cracked, ULTRA information was only distributed to flag officers. McVay was not of this rank, and therefore did not receive this crucial information before sailing to Leyte.

USS Indianapolis

The USS Indianapolis was a US naval ship, a heavy cruiser, which was torpedoed and sunk early in the morning of July 30th, 1945.