Spies and Lies: Tricks of the Trade in the Civil War by Tom Rizzo

The Union spy blinked a few times to clear his eyes of drifting candle smoke, and removed the toothpick soaking in a shallow bowl of lemon juice. Satisfied it was saturated enough, he began scratching an invisible message into the vellum paper on the table in front of him, taking care to keep it away from the flame. 

Minutes later, he exhaled, relieved to be finished. He snapped the toothpick in half and dropped the two pieces inside one of his boots. The fingers of his right hand ached from having to write so deliberately, and wiggled them open and closed a few times. He stared at the paper, squinted, and shrugged, not knowing whether the secret communique' was even legible. 

One of his colleagues had described the plodding activity as "writing in the blind." The message could only be displayed when the recipient held it in front of a candle flame.  

Messages written in invisible ink — by both the North and the South — provided key information to military commanders, including: locations of regiments, artillery batteries, prisoners and supplies.

The shorter the message, the better. Simple trumped complex.

Creating messages with lemon-juice, and other invisible ink techniques, were effective, but couldn't be used with much success in communicating more complex information, such as detailed battle plans.

Invisible writing, however, was among the chief techniques spies on both sides used to convey coded messages. But savvy code-breakers were usually able to decipher the messages, which often revealed the location or tactics of an enemy army.

Spying played a key role in the American Civil War for both sides, and in different ways.

Elizabeth Van Lew,  a spy for the Federals, for example, wrote her messages in a special ink that would only be revealed when milk was applied to them.

Confederate spy Rose O'Neal Greenhow  wouldn't allow her own arrest and imprisonment deter her undercover efforts. In fact, she managed to devise encoded  messages in letters to fellow spies in a most ingenious way.

Knowing that her letters would be scrutinized, Greenhow commandeered a bottle of invisible ink solution. But, rather than use it, she placed the bottle where she knew Union authorities would discover it. And, they did. As a result, detectives wasted valuable time checking her letters for invisible ink rather than deciphering the hidden messages in her supposedly innocent correspondence.

For some operatives, the work was fairly routine, and involved debriefing runaway slaves, deserters and others.

The telegraph played a key role in spy activity.

When Union agents managed to tap into the Confederate telegraph line between Chattanooga and Knoxville, for example, they intercepted mail but, at the same time, transmitted phony messages to confuse the enemy even further.

At one point, Confederate Commander Robert E. Lee expressed nothing but distrust for the telegraph, and once prohibited his officers from using the device.

Union Commander Ulysses S. Grant, on the other hand, used the military telegraph to implement key tactics and important strategy. From his headquarters, Grant issued daily orders by telegraph and received reports regarding the operations and movement of the various armies under his command. Grant was able to utilize the telegraph to maintain direct control over half a million soldiers, stretched across a territory of 800,000 square miles.

"The value of the telegraph cannot be exaggerated, as illustrated by the perfect accord of action of the armies of Virginia and Georgia," said General William T. Sherman.

Both North and South engaged in a broad variety of spying activity to unearth vital military intelligence.

A variety of spying techniques was put into play to gain the upper hand, knowing that one, vital piece of information that could make the difference between victory and defeat.

In addition to a vast network of relatively inexperienced intelligence agents, both armies tried new tactics.

New technology, such as photography, came into play. Soliders of either army were always willing to pose for photographs, which were studied for information on the terrain and location of an enemy army.

Both sides took to the air with hot-air or hydrogen-filled balloons for spy missions.

On September 24, 1862, Union spy Thaddeus Lowe rose to more than 1,000 feet where he spotted movement by Confederate troops. He transmitted his discovery to Union  soldiers on the ground who began firing on the Rebels.

During the Battle of Fiar Oaks, Lowe issued hourly transmissions on Confederate troop movements. The information ranked as a key factor in the Union's victory.

In 1862, the Confederate spies went aloft in hot air balloons so they could sketch the Union's positions in Yorktown, Virginia.

While aloft, balloonists used telegraph or signal flags to transmit the information they gathered. Some Union Army balloons were sizable enough to accommodate telegraphers and their gear so there would be no delay in relaying their observations to officers on the ground.

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