The Ten Thousand (Ancient Greek: , oi Myrioi) were a force of mercenary units, mainly Greeks, employed by Cyrus the Younger to attempt to wrest the throne of the Persian Empire from his brother, Artaxerxes II. Their march to the Battle of Cunaxa and back to Greece (401-399 BC) was recorded by Xenophon, one of their leaders, in his work Anabasis.
Xenophon stated in Anabasis that the Greek heavy troops routed their opposition twice at Cunaxa at the cost of only one Greek soldier wounded. Only after the battle did they hear that Cyrus had been killed, making their victory irrelevant and the expedition a failure.
The Ten Thousand found themselves far from home with no food, no employer, and no reliable allies. They offered to make their Persian ally Ariaeus king, but he refused on the grounds that he was not of royal blood and would not find enough support among the Persians to keep the throne. They offered their services to Tissaphernes, a leading satrap of Artaxerxes, but he demanded their complete surrender, which they refused. This presented Tissaphernes with a problem - a large army of heavily-armed troops, which he could not defeat by frontal assault. He supplied them with food and, after a long wait, led them northwards for home. Meanwhile he succeeded in luring away the Persian general Ariaeus and his light troops.
The Greek senior officers accepted the invitation of Tissaphernes to a feast where they were taken prisoner, led before the king, and decapitated. The Greeks elected new officers, among them Xenophon, and set out to march northwards to the Black Sea, through Corduene and Armenia.
Xenophon and his men initially had to deal with volleys from a minor force of harassing Persian missile cavalry. Every day, this cavalry, finding no opposition from the Ten Thousand, moved cautiously closer and closer. One night, Xenophon formed a body of archers and light cavalry. When the Persian cavalry arrived the next day, now firing within several yards, Xenophon suddenly unleashed his new cavalry in a charge, smashing into the stunned and confused enemy, killing many and routing the rest. Tissaphernes pursued Xenophon with a vast force, and when the Greeks reached the wide and deep Great Zab river, they seemed to be surrounded. However, Xenophon quickly devised a plan; all goats, cattle, sheep, and donkeys were slaughtered and their bodies stuffed with hay, laid across the river, and sewed up and covered with soil so as not to be slippery. This created a bridge across which Xenophon led his men before the Persians could get to them. That Xenophon was able to feed his force in the heart of a vast empire with a hostile population was astonishing. Dodge notes:
On this retreat also was first shown the necessary, if cruel, means of arresting a pursuing enemy by the systematic devastation of the country traversed and the destruction of its villages to deprive him of food and shelter. And Xenophon is moreover the first who established in the rear of the phalanx a reserve from which he could at will feed weak parts of his line. This was a superb first conception.
The Ten Thousand eventually made their way into the land of the Carduchians, a wild tribe inhabiting the mountains of modern southeastern Turkey,
"a fierce, war-like race, who had never been conquered. Once the Great King had sent into their country an army of 120,000 men, to subdue them, but of all that great host not one had ever seen his home again." 
The Ten Thousand made their way in and were fired at with stones and arrows for several days before they reached a defile where the main Carduchian host stood. In the Battle of the Carduchian Defile, Xenophon had 8,000 men make a diversionary attack on this host whilst he marched the other 2,000 under cover of a rainstorm to a pass revealed by a prisoner, and
"having made their way to the rear of the main pass, at daylight, under cover of the morning mist, they boldly pushed in upon the astonished Carducians. The blare of their many trumpets gave notice of their successful detour to Xenophon, as well as adding to the confusion of the enemy. The main army at once joined in the attack from the valley side, and the Carducians were driven from their stronghold."
After heavy mountain fighting in which Xenophon showed the calm and patience needed for the situation, the Greeks made their way to the northern foothills of the mountains at the Centrites River, only to find a major Persian force blocking the route north. With the Carduchians surging toward the Greek rear, Xenophon again faced the threat of total destruction in battle. Xenophon's scouts quickly found another ford across the river, but the Persians moved and blocked this as well. Xenophon, in a cunning tactic similarly repeated by Robert E. Lee before the Seven Days Battles, sent a small force back toward the other ford, causing the anxious Persians to detach a major part of their force. Xenophon stormed and completely overwhelmed the force remaining at his ford, while the Greek detachment made a forced march to this bridgehead. This was among the first attacks in depth ever made, 23 years after Delium and 30 years before Epaminondas' more famous use of it at Leuctra.
Winter had by now arrived as the Greeks marched through Armenia "absolutely unprovided with clothing suitable for such weather", inflicting more casualties than they suffered at their skillful ambush of a local satrap's force and the flanking of another force. At a stage when the Greeks were in desperate need of food, they decided upon attacking a wooden castle known to have provisions. The castle, however, was stationed on a hill surrounded by forest. Xenophon ordered small parties of his men to appear on the hill road; and when the defenders fired boulders, a soldier would leap into the trees, and he "did this so often that at last there was quite a heap of stones lying in front of him, but he himself was untouched." Then, "the other men followed his example, and made it a sort of game, enjoying the sensation, pleasant alike to old and young, of courting danger for a moment, and then quickly escaping it. When the stones were almost exhausted, the soldiers raced one another over the exposed part of the road", storming the fortress, which, with most of the garrison now neutralized, barely put up a fight.
Xenophon records the joyful moment when the Ten Thousand (by then actually far fewer), from the heights of Mount Theches, saw the sea and friendly Greek colonies on the coast, which signified their escape had been made good, whereupon they shouted Thalatta! Thalatta! ("The sea! The sea!"). Soon after, Xenophon's men reached Trapezus on the coast of the Black Sea (Anabasis 4.8.22). Before they departed, the Greeks made an alliance with the locals and fought one last battle against the Colchians, vassals of the Persians, in mountainous country. Xenophon ordered his men to deploy their line extremely thin, so as to overlap the enemy, while keeping a strong reserve. The Colchians, seeing they were being outflanked, divided their army to check the Greek deployment, opening a gap in their line through which Xenophon rushed in his reserves, scoring a brilliant Greek victory.
On their arrival at Trapezus on the Euxine, the Greek mercenaries sent their Spartan general Cheirisophus to Anaxibius, the Spartan admiral stationed at Byzantium in 400 BC, to obtain a sufficient number of ships to transport them to Europe. However, when Cheirisophus met them again at Sinope, he brought back nothing from Anaxibius, but civil words and a promise of employment and pay as soon as they came out of the Euxine.
The Ten Thousand under Xenophon continued to the west, some by ship, but most of them by land, and arrived in Bithynia after numerous skirmishes and plunderings. Pharnabazus, satrap of Hellespontine Phrygia, was involved in helping the Bithynians against these plundering raids of the Ten Thousand. He was also trying to stop them from entering Hellespontine Phrygia. His cavalry, which made several raids on the Greek mercenaries, is said to have killed about 500 of them.
Pharnabazus then arranged with the Spartan Anaxibius for the rest of the Ten Thousand to be shipped to Byzantium. On their arrival at Chrysopolis, on the Asiatic shore of the Bosporus, Anaxibius, being bribed by Pharnabazus with great promises to withdraw them from his satrapy, promised to pay them and brought them over to Byzantium. Here Anaxibius attempted to send them forward on their march without fulfilling his agreement. A fight ensued, in which Anaxibius was compelled to flee for refuge to the Acropolis, and which was quelled only by the remonstrances of Xenophon. Soon after this, the Greeks left the town under the command of the adventurer Coeratades; and Anaxibius issued a proclamation, subsequently acted on by the harmost Aristarchus, that all of Cyrus's soldiers found in Byzantium should be sold as slaves.
Filled with many instances of originality and tactical genius, Xenophon's conduct of the retreat caused Dodge to name the Athenian knight the greatest general to precede Alexander the Great.
According to Xenophon, the Ten Thousand were composed of:
In addition, they were backed up by a fleet of 35 triremes under Pythagoras the Spartan and 25 triremes under Tamos the Egyptian, as well as 20,000 Persian troops under Ariaeus the Persian. (Although Xenophon lists them as 100,000, most modern historians believe Ariaeus' troops numbered only about 20,000).
Until shortly after the Battle of Cunaxa, Spartan general Clearchus was recognized as the commander of the army. When Tissaphernes arrested and executed Clearchus, Proxenus, Menon, Agias (possibly the same person as Sophaenetus), and Socrates, their places were taken by Xenophon the Athenian, Timasion the Dardanian, Xanthicles the Achaean, Cleanor the Orchomenian, and Philesius the Achaean, with the Spartan Cheirisophus as the general commander.
When the Ten Thousand started their journey in 401 BC, Xenophon stated that they numbered around 10,400. At the time Xenophon left them two years later, their number had dwindled to just under 6,000.