Via Bruce Schneier, a retired CIA codebreaker recently decoded a message sent to Confederate Lt. Gen. John Pemberton in July 1863:
The encrypted, 6-line message was dated July 4, 1863, the date of Pemberton's surrender to Union forces led by Ulysses S. Grant, ending the Siege of Vicksburg in what historians say was a turning point midway into the Civil War.
The message is from a Confederate commander on the west side of the Mississippi River across from Pemberton.
"He's saying, 'I can't help you. I have no troops, I have no supplies, I have no way to get over there,'" Museum of the Confederacy collections manager Catherine M. Wright said of the author of the dispiriting message. "It was just another punctuation mark to just how desperate and dire everything was."
That day, 4 July 1863, the Union not only captured Vicksburg but also prevailed at Gettysburg. Historians generally agree the two victories effectively ended any possibility of the Confederacy winning the war, though they would continue to fight for another 20 months.
The full text of the message to Pemberton reads:
You can expect no help from this side of the river. Let Gen'l Johnston know, if possible, when you can attack the same point on the enemy's lines. Inform me also and I will endeavor to make a diversion. I have sent some caps (explosive devices). I subjoin a despatch from General Johnston."
The last line, Wright said, seems to suggest a separate delivery to Pemberton would be the code to break the message.
The news story has more details about how they found the message, and how they broke the code.