The glass vial stopped with a cork contained a coded missive to Lt Gen John C Pemberton, who was besieged in the Mississippi city by Union forces led by Ulysses S Grant. After six weeks, people in Vicksburg had resorted to eating cats, dogs and leather, and making soup from wallpaper paste.
The encrypted six-line message was dated July 4, 1863, the date of Pemberton's surrender, and would have offered no hope to him. It said: "You can expect no help from this side of the river," 'The Daily Telegraph' online reported.
The source of the message is thought to have been Maj Gen John G Walker, of the Texas Division. Pemberton had also held out hope that General Joseph E Johnston, and his 32,000 Confederate troops camped south of Vicksburg, would eventually come to his aid, but message said that's not going to happen.
Catherine Wright, collections manager at the Museum of the Confederacy, said: "He's saying, 'I can't help you. I have no troops, I have no supplies, I've no way to get over there.' It was just another punctuation mark to just how desperate and dire everything was."
The bottle also contained a bullet, which was thought to have been to weigh it down if the messenger was caught and had to throw it in a river.
It had been kept in the museum since 1896 before being opened. The message initially appeared to be a random collection of letters but was deciphered after several weeks by David Gaddy, a retired CIA code breaker.
It had been drawn up using the Vigenere cipher, which involves shifting letters of alphabet a set number of places.