Top secrets come spilling out

In 1917, British analysts deciphered a coded message the German foreign minister sent to one of his country's diplomats vowing to begin "unrestricted submarine warfare" and seeking to win over Mexico with a promise to "reconquer the lost territory in Texas, New Mexico and Arizona" if the US entered the world war. When it became public, the Zimmerman Telegram caused a sensation, helping propel the US into the conflict against Germany.