Navy codebreakers faked a water shortage to confirm Midway

By May 1942, Commander Joseph Rochefort's codebreaking unit at Station HYPO had partially broken Japan's naval code and could see repeated references to an upcoming operation against a target designated only as 'AF.' Rochefort was confident AF meant Midway Atoll, based on patterns in the intercepted traffic, but his superiors in Washington weren't persuaded by pattern analysis alone — and the decision at stake was not trivial: where to position the entire US Pacific carrier fleet against a Japanese armada that outnumbered it.

The standard options were both bad. Waiting for more intercepted traffic to build overwhelming statistical confidence risked running out of time before the Japanese fleet sailed. Acting on Rochefort's confident but essentially circumstantial inference risked deploying America's remaining carriers to the wrong location entirely, based on a codebreaker's judgment call that Washington had explicitly said wasn't convincing enough to bet on.

Had Midway transmit a fake uncoded distress call about a water shortage, then watched for Japan to relay it in their own coded traffic, confirming the target on command.

The fake distress message was plausible and routine, so Japan relayed it as real intelligence.

The planted detail was arbitrary, so its exact reappearance proved a real information chain.

The test used the enemy's own relay behavior as an instrument, needing no new intelligence source.

It resolved the ambiguity quickly, before the Japanese fleet sailed.

When you hold a high-confidence inference from indirect signals, test it cheaply by seeding a distinctive false detail into the observed channel and watching for its echo.

The confirmation let Admiral Chester Nimitz commit his three available carriers to an ambush position northeast of Midway with confidence rather than a hedge, instead of splitting his forces or guessing wrong. The US Navy sank four Japanese fleet carriers at the Battle of Midway (June 3–7, 1942) for the loss of one of its own, a result now widely regarded as the turning point of the Pacific War.