1780: Treasonous Ciphered Letter
During the American Revolution, British Major John André joined with American General Benedict Arnold in a scheme to secure British control over the American fortification at West Point, New York. Before it could be carried out, however, minutemen captured John André and informed General George Washington of the plot. Arnold managed to evade Washington's arrest warrant, but André was imprisoned at Tappan, New York, and on September 29, 1780, he was found guilty of being behind American lines "under a feigned name and in a disguised habit. He was executed as a spy by Washington's order October 2, 1780.
(Image and portions of text borrowed from www.mountvernon.org)