Battle of Jutland
That was because the British had gotten hold of three codebooks from wrecked ships. One code, for smaller vessels, was from a German freighter. Another, for diplomatic messages, was taken from a wreck on the bottom of the North Sea. The code for the German High Seas Fleet was from a cruiser that was wrecked in the Baltic. The British did not make the best use of thse sources, but they had in their hands a priceless advantage.