Throughout the early summer of 1863, Union Army and Navy units under MG Ulysses Grant and Admiral David Porter laid siege to Vicksburg, one of the last Confederate strongholds on the Mississippi River.
During the siege, the Confederates had insight into Union movements because they were copying Union flag signals and solving the cipher system that protected messages. All too often, however, the inside knowledge had little effect because the besieged Confederates did not have the force available to take appropriate actions.
A Confederate position on the Devil’s Backbone, a hill north of the city, allowed signalmen to observe the Union signal station and copy messages sent out by flag. At this time, the Confederates had solved the Union cipher system and could read the messages. Commander of the observation position was Mathew H. Asbury, a Signal Corps officer from Louisiana.
In early July, fighting was suspended while Grant considered the latest proposal concerning capitulation from General John C. Pemberton, commander of the garrison inside Vicksburg.
On the night of July 3, a message was sent from Grant’s headquarters to Admiral Porter. (The visual signal system used flags in daylight, torches at night.)
The message said that the Union council of generals had recommended that all personnel captured at Vicksburg be sent north to prison camps. However, Grant had decided that he could not spare enough guards and transport ship to do this. He also believed that the Vicksburg garrison was so demoralized that its men, if paroled, would spread dissatisfaction wherever they went throughout the south.
Inside Vicksburg, this secretly acquired knowledge that they would be spared imprisonment apparently made the difference in the decision to give up the city.
Pemberton decided to surrender Vicksburg on July 4. He and his men were paroled, but they gave up a large store of weaponry to the Union forces.
Disgraced, Pemberton was never given another command as a general officer. He accepted an assignment as a lieutenant colonel in an artillery unit. A native of Pennsylvania, Pemberton returned to his home state after the war.
The key message on July 3 was genuine; that is, it gave a true account of the decision by Grant to reject the recommendation of his senior officers. Looking at the situation in the light of the modern experience of COMINT, however, one wonders if it might have been sent knowingly, with a recognition that the Confederates could read Union signals and with the hope that it would push the Confederates into surrender. This is nowhere confirmed in available records and remains nothing more than speculation.