"On This Date in History" Calendar
1777: Benedict Arnold overlooked for promotion.
19 February 1977: Congress overlooks Benedict Arnold, the hero of Saratoga and Ticonderoga, for promotion to major general in favor of five other junior officers. Infuriated, Arnold ends up making his traitorous deal to sell the garrison of West Point to the British. He used a dictionary code in his correspondence to British in which he proposed the agreement.
Read about the whole ordeal on pages 24 - 30 of the NSA publication "Revolutionary Secrets: Cryptology in the American Revolution" linked below.