Everyone loves a spy story. They are thrilling tales of daring undercover agents in a cat-and-mouse game against other shadowy organizations trying to stop them. Today, most espionage involves the tapping of phones and the interception of electronic information. But in the past, things were done a lot differently. Humans are a combative species, and as long as one group has warred against another, they have tried to use underhanded tactics to get the upper hand.
Here are ten ways that spying was employed in the ancient world.
Related: 10 of the Most Ingenious Deception Tactics Used in War
10 The 12 Spies
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Spying is a dirty business and not usually associated with godliness. The Bible mentions several times when spies were used by the forces of God. In the Book of Numbers, we are told that Moses sent out 12 spies to scout out the land of Canaan, which God had promised to the Israelites. It is God, in fact, who orders this reconnaissance. “And there the Lord spoke to Moses, saying: Send men to view the land of Chanaan, which I will give to the children of Israel, one of every tribe, of the rulers.”
The Bible gives the names of the 12 men sent out to spy on their new territories. They find that it is the land flowing with milk and honey that they were promised. They bring back lush fruits to show its bounty. But they also report that they had seen well-fortified towns and say, ” No, we are not able to go up to this people, because they are stronger than we.”
The spies then begin to tell the Israelites that the people who already live there are like giants and that the land is harsh to discourage them from invading. They rebel against Moses and speak of going back to Egypt. God, as is his way in the Old Testament, takes this badly. He threatens to kill them all with a plague, but Moses is able to talk him down. God softens and merely tells those who spoke against him, “Your carcasses shall lie in the wilderness.” He then punishes the rest by forcing them to wander in the desert for 40 years.
In the world of espionage, the act of concealing a message in another object is known as steganography. Sometimes, the object used to hide information is a living person.
The Greek author Herodotus says that Histiaeus, tyrant of Meletus under the Persian kings, used a novel method of sending his secret letters. Histiaeus was never the most loyal of servants to the king and was always on the lookout for a chance to seize full power for himself. When he began to fear that his lands would be taken from him, he was at the royal court and needed to get a message out to his allies that they should rebel, but all of his mail was searched.
To get around anyone reading his seditious letters, he took one of his slaves, shaved him, and tattooed the real message on the slave’s head. He then let the slave’s hair grow to cover it up. The messenger was sent out with an innocuous letter, but when he delivered it, he told the recipient to shave his head. The real message was read and sparked what became known to history as the Ionian Revolt, which sparked generations of war between the Greeks and the Persians. 
The Trojan War myths of ancient Greece feature one of the greatest acts of subterfuge in all of history. The Greeks, after many years of laying siege to Troy, finally breached the walls by pretending to give up, leaving a giant wooden horse outside. When the Trojans take it inside, a group of warriors concealed inside sneak out and open the gates. Troy is destroyed. Homer’s epic poem The Iliad is a story full of heroes, the pursuit of glory, and brutal hand-to-hand fighting. But its 10th book follows a Trojan spy called Dolon.
The Greeks, on the whole, did not think much of spies. Dolon is described as being very ugly, as opposed to the idealized beauty of the heroes. To spy on the Greek ships, Dolon crawls towards their camp wearing grey wolf skin. Greek vase paintings often show him in this disguise with his bushy beard poking out.
Alas, Dolon was spotted by Odysseus and Diomedes. The captured spy immediately begins to reveal everything. After informing the Greeks about which Trojans are camped where, the captors have no further use for him and promptly decapitate the spy.
7 Ovid’s Milk
Not all subterfuge is employed in warfare. Sometimes, lovers need help in concealing their messages. The Roman poet Ovid had some advice for young ladies who wanted to pass love letters to their partners without any meddling family members seeing them.
In his epic poem The Art of Love, Ovid describes how you can write a message with fresh milk that will be invisible to the naked eye. To read it, all you have to do is brush the surface with soot, which will stick to the hidden words.
If you find your chastity too closely guarded by your family, Ovid has other tips for passing secret messages.
“When a knowing maid can carry letters you’ve penned,
concealed in the deep curves of her warm breasts?
When she can hide papers fastened to her calf,
or bear charming notes tied beneath her feet?
The guard’s on the look-out for that, your go-between
offers her back as paper, and takes your words on her flesh.”
So, if you don’t have milk, just use your servant’s skin.
6 Kill the Messenger
Shooting the messenger is a bad idea. If you punish those who bring you bad news, then you will only be left with “yes men” around you. Even the ancients knew this. Plutarch recalls how Tigranes, at war with the Romans, fell victim to this tendency. “The first messenger was so far from pleasing Tigranes that he had his head cut off for his pains; and no man dared to bring further information. Without any intelligence at all, Tigranes sat while war was already blazing around him, giving ear only to those who flattered him.”
Some have tried to use a shoot-the-messenger technique on their own messengers, and that did not work out either. Pausanias was a 5th-century BC regent of Sparta. He had it in him to be a great leader, but he was always untrustworthy. He came undone when he tried to raise a revolt with the help of the Persians. He sent a messenger to the Persian king bearing a letter. To ensure the message stayed secret, the letter ended with the instruction, “Kill the man who bears this message.”
The messenger read the letter before delivering it and was understandably annoyed. He reported the secret dealings between Pausanias and the Persians to the Spartans. Pausanias was bricked up alive in a temple until he starved to death.
5 Sun Tzu’s Spies
The Art of War is revered by many as not only a guide to carrying out a successful war but also a successful life. Written in the 5th century BC in China by Sun Tzu, it has influenced readers from General Douglas MacArthur to the Soviet KGB. In the work’s final chapter, Sun Tzu describes how best to use spies.
Sun Tzu describes five types of spy: the local spy, the inside spy, the reverse spy, the dead spy, and the living spy. Local spies are ones that you hire from an enemy city. Inside spies are members of the opposing military that you can bribe. The reverse spy is an enemy spy you catch and convince to work for you. The living spy is one you send into enemy territory and returns to you with information. The dead spy is perhaps the most interesting.
By placing false information you want the enemy to find, you can place it on a person’s dead body. This type of subterfuge was used by the British in World War II during Operation Mincemeat when they placed fake intelligence documents on a corpse and set it afloat off the coast of Spain.
4 Hannibal’s Wigs
The Carthaginian general Hannibal is renowned for his campaign, which took on the full might of the Roman state. Hannibal carried his army all the way into Italy and came within a hair’s breadth of capturing Rome itself. His techniques were often not subtle. You do not march elephants over the Alps to wage a secret war. But Hannibal was not beyond using spy techniques when he needed to.
When Hannibal was crossing the territory of the Celts, he realized that they might try to assassinate him. The writer Polybius described the lengths he went to in order to save himself. “He caused a number of wigs to be made for him, suited in appearance to men of various ages; and these he constantly varied, changing at the same time his clothes also to harmonize with the particular wig which he wore. He thus made it hard to recognize him, not only for those who met him suddenly, but even for his intimates.”
This is a hairy part of Hannibal’s warfare that is usually ignored by historians.
3 Aeneas Tacticus’s Secret Messages
The ancient Greek world was one of multiple city-states that were almost always at war with one another. Given the Greek propensity to study everything, it was only natural that they would also turn their minds to the examination of war. One of the first writers to do this lived in the 4th century and was called Aeneas Tacticus. His sole surviving work is called How to Survive Under Siege and describes the steps one needs to take to defend a fortified city.
Tacticus says that one must find ways of getting messages out of the city to signal to your allies for help and tells you a number of ways to do so. You could take any book and use a pin to poke minute holes next to certain letters to spell out words. Even more secret methods can be used. He suggests sewing the message into the heel of your messenger’s shoes. Just in case it rains, he says, you should inscribe the message on a thin piece of tin. If you use thin pieces of lead to write your message, you can turn them into earrings and send them out on the ears of women.
One method requires an injured man. At a siege in Ephesus, a message was written on leaves that were used to wrap up the wound on a man’s leg. When he was allowed out of the city to recuperate, he carried the message with him.
To survive a siege, you have to be able to not only send out the messages you want to escape but also stop anyone you don’t want talking to the enemy getting one out. Aeneas Tacticus tells you how to watch out for treachery from within by giving examples of how cities have been betrayed before.
At one siege, the man who was set to guard the gate was allowed out of the city to fill a jar with water. While doing this, he would tell the enemy during which watch of the day he would be in charge of the gate. He would leave one stone by the well if he was on the first watch, two if the second watch, etc. The enemy then used this information to attack the gate when they knew the traitor would open it for them.
Tacticus says that once the city is under siege, the leader should lock the gate and not let anyone out.
1 Message on an Arrow
Getting a message into a city under siege can be tricky. Why not use a weapon of war to get your secret message through?
At the siege of Potidaea in 480 BC, the Persian leader Artabazus tried to get help from a traitor inside the walls by wrapping his messages around the shaft of an arrow and shooting it at the city. Unfortunately for the Persians, this was not a foolproof method of delivering secrets. One of the arrows with a message struck the shoulder of someone standing on the walls. Once the arrow was pulled out, the message was noticed, and the conspiracy was uncovered.
When Quintus, brother of Cicero, was under siege in Gaul, he tried to get a message to Caesar calling for help. All his first messengers were caught and killed, so Quintus wrote his letter on the shaft of a javelin, which was carried out of the camp without being detected. Caesar replied in the same way. His message saying that help was on the way was also attached to a javelin and thrown back into camp. This was not foolproof either. The javelin was stuck in a wall, and no one noticed the message for two days.