AF is short of water
"Af is short of water" turned the tide for the American forces battling on the Pacific Front in WWII. In the spring of 1942, Allies had already cracked the Japanese Navy cipher JN-25, and had intercepted Japanese plans to attack a US base on island "AF." Intelligence knew that AF referred to a base in the Pacific, but were uncertain which one. Backing up the wrong island would be catastrophic, so to narrow the field, the Americans sent out a fake transmission stating that the water distillery on Midway island was damaged, and the base was running out of fresh water. The transmission was intercepted by the Japanese and the communique "AF is short of water." was sent out, identifying Midway as the location the attack was planned. On June 4th, 1942 the Japanese lost 4 carriers to a fleet of American ships that had been ready and waiting for the attack. It was the turning point for the War in the Pacific, and all because a code was broken.
from Decoding the Universe