“… deciphered the rebels’ new code of signals.”: SIGINT during the Atlanta Campaign

I’ve always felt the Signal Corps does not get due attention for its work during the Civil War.  Maybe that is just the opinion of an old Army signal-soldier showing through.  However, when’s the last time you saw a full handling of the signal work as part of a campaign study?

The Atlanta Campaign offers an excellent example of the work done by the signal troops (as does the Overland Campaign and Petersburg Siege… and I have some writing chores in that regard!).  And among the formations constituting the armies of Major-General William T. Sherman’s Division of Mississippi, the activities of those supporting the Army of Tennessee (Major-General Jame McPherson) stand out – perhaps mostly because they are well documented by reports filed at the time.  Captain Ocran H. Howard served as Chief Signal Officer for that field army.  A signal detachment served with each corps within that army.  Lieutenant Samuel Edge led the detachment assigned to the Fifteenth Corps (Major-General John A. Logan).

Allow me to focus narrowly, for this post, on work of Edge’s detachment over the first half of June 1864.  From his report of the campaign:

June 1, Lieutenants Edge and [Samuel] Sample moved to the front of the new position of the Army of the Tennessee and established a station of observation and received several contraband dispatches and transmitted them to Generals McPherson and Logan as soon as received. Also gave general information of the enemy moving from our front. June 2, received several more contraband dispatches and transmitted them promptly to the generals commanding.

“Contraband dispatches”… no, not messages from escaped slaves.  Edge and Sample were “reading” the Confederate wig-wags.  Edge went on to mention his detachment received four lieutenants and twenty-eight men as reinforcements on June 3.  And those men were quickly brought up to speed on the methods of “stealing” Confederate messages.  Then on June 5, Edge and Sample turned their game up a notch:

June 5, at daylight Lieutenants Edge and Sample found the enemy’s works evacuated, and proceeded immediately to old rebel signal station. Answered rebel signal call from Lost Mountain, received contraband message, which was promptly transmitted to Major-Generals McPherson and Logan.

This bold move put the Federal signal operators in the vicinity of Pine Mountain and enabled the infantry to better feel out the new Confederate position.  The detachment, along with the rest of the Army of Tennessee moved towards Acworth, closing on the next Confederate.  In the new Federal position, Edge’s men established communications with a station on Allatoona Mountain, as part of the chain of Federal links going back to Chattanooga.  Edge’s detachment maintained a position in a white frame church, and were mostly involved with communications within Federal lines.  While in that position, Sample, Edge’s very capable subordinate, moved over to support Seventeenth Corps.  On June 10, Edge and his detachment moved forward with the Fifteenth Corps and resumed their “spying” on Confederate lines… but they soon encountered a problem:

June 10, abandoned station on church and moved with the Fifteenth Army Corps, by order of the commanding general, to Big Shanty. Lieutenant Edge, with the assistance of the officers in the detachment, established two stations of observation, received two contraband dispatches, and transmitted them to the commanding generals as soon as received, when the rebels changed their code and for one day cut off our contraband information.

So how long does it take to crack the codes?

June 11, watched rebel movements all day. The officers, with the aid of Capt. O.H. Howard, chief signal officer, deciphered the rebels’ new code of signals. Lieutenant Edge received two contraband messages and found them complete.

Keep in mind the encoding system in use on the Confederate side – something similar to the code wheels used by Federals or perhaps just a derivation of Beauregard’s matrix.  With an intercepted string of text, one can look for patterns.  But a good sample set is required.

With the Confederate code broken, Edge and his men went to work gleaning information from the Confederate signal stations along the line of defense centered on Lost Mountain:

June 14, moved to the front of Kenesaw Mountain, and established two stations of observation. Lieutenants Edge, [Isaiah C.] Worley, and [William W.] Allen occupying one, and Lieutenants [John H.] Weirick and [Charles H.] Fish the other, received several contraband messages of considerable importance, which were transmitted with promptness to Major-Generals McPherson and Logan. June 15, occupied the same stations; received several contraband messages, all of which were transmitted to the generals…. June 16, occupied same stations and gave important information to the generals.

But not all the signal activity involved capturing Confederate messages.  They assisted the artillery while there:

Lieutenant Weirick directed the firing of the First Minnesota Battery, Captain Clayton, by the aid of his glass, which resulted in blowing up a caisson and knocking off one wheel of a gun.

On June 17, the Confederates fell back from the Lost Mountain line.  The signal stations gave early indications of this movement… and for good measure, harassed the rebels in their retreat:

June 17, Lieutenant Edge noticed the enemy making movements indicating an evacuation, and reported the same to the generals; also assisted, with the aid of his glass, the firing of two batteries.

When the Federals moved to occupy the former Confederate works, the signal detachment advanced to resume their work against the Confederate signal stations:

June 18, the enemy had evacuated their works. Lieutenant Edge went to the front and received two contraband messages, and handed them over to Maj. Gen. W. T. Sherman, who was standing on the spot. Our batteries then drove the rebel signal officers off the mountain, and there has not been a rebel signal station seen by any officer in the department since.

So somewhat abruptly, the Federal signals intelligence (SIGINT in our modern acronym filled military vocabulary) came to an end.  The official records contain seven full pages of captured dispatches.  Many of these are detailed to the point of naming commanders and their dispositions.  Valuable information given directly to Sherman and his staff at a critical juncture of the campaign.

We see a lot of spy stories associated with the Civil War.  Lots of tales involving hoop skirts and night rides.  But as seen with the Winter Encampment in Virginia, much of the intelligence gathering was done by less romantic methods – more often than not involving wig-wag stations.

More to follow about Edge’s exploits during the Atlanta Campaign…

(Edge’s report of the campaign appears in OR, Series I, Volume 38, Part III, Serial 74, pages 119-123.)

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