The Achaemenid Empire of Persia (550 - 330 BCE) had an elite corps of heavy infantry that was so effective, it helped them to conquer much of the known world. These troops also served as the imperial guard. We have beautiful depictions of them from the walls of the Achaemenid capital city of Susa, Iran, but unfortunately, our historical documentation about them comes from the Persians' enemies -- not really an unbiased source.
Herodotus, Chronicler of the Persian Immortals
Chief among the chroniclers of the Persian Immortals is the Greek historian Herodotus (c. 484 - 425). He is the source of their name, in fact, and it may be a mistranslation. Many scholars believe that the actual Persian name for this imperial guard was anusiya, meaning "companions," rather than anausa, or "non-dying."
Herodotus also informs us that the Immortals were maintained at a troop strength of exactly 10,000 at all times. If an infantryman was killed, sick, or wounded, a reservist would immediately be called up to take his place. This gave the illusion that they were truly immortal, and could not be injured or slain. We do not have any independent confirmation that Herodotus's information on this is accurate; nevertheless, the elite corps is often referred to as the "Ten Thousand Immortals" to this day.
The Immortals were armed with short stabbing spears, bows and arrows, and swords. They wore fish scale armor covered by robes, and a headdress often called a tiara that reportedly could be used to shield the face from wind-driven sand or dust. Their shields were woven out of wicker. Achaemenid artwork shows the Immortals decked out in gold jewelry and hoop earrings, and Herodotus asserts that they wore their bling into battle.
The Immortals came from elite, aristocratic families. The top 1,000 had gold pomegranates on the ends of their spears, designating them as officers and as the king's personal bodyguard. The remaining 9,000 had silver pomegranates. As the best of the best in the Persian army, the Immortals received certain perks. While on the campaign, they had a supply train of mule-drawn carts and camels that brought along special foods reserved only for them. The mule train also brought along their concubines and servants to tend to them.
Like most things in the Achaemenid Empire, the Immortals were equal opportunity -- at least for elites from other ethnic groups. Although the majority of the members were Persian, the corps also included aristocratic men from the previously-conquered Elamite and Median Empires.
The Immortals at War
Cyrus the Great, who founded the Achaemenid Empire, seems to have originated the idea of having an elite corps of imperial guards. He used them as heavy infantry in his campaigns to conquer the Medes, the Lydians, and even the Babylonians. With his last victory over the new Babylonian Empire, at the Battle of Opis in 539 BCE, Cyrus was able to name himself "king of the four corners of the world" thanks in part to the efforts of his Immortals.
In 525 BCE, Cyrus's son Cambyses II defeated the Egyptian Pharaoh Psamtik III's army at the Battle of Pelusium, extending Persian control across Egypt. Again, the Immortals likely served as the shock troops; they were so feared after their campaign against Babylon that the Phoenicians, the Cypriots, and the Arabs of Judea and the Sinai Peninsula all decided to ally themselves with Persians rather than fighting them. This left the door to Egypt wide open, in a manner of speaking, and Cambyses took full advantage of it.
The third Achaemenid emperor, Darius the Great, likewise deployed the Immortals in his conquests of Sindh and parts of the Punjab (now in Pakistan). This expansion gave the Persians access to the rich trading routes through India, as well as the gold and other wealth of that land. At that time, the Iranian and Indian languages were probably still similar enough to be mutually intelligible, and the Persians took advantage of this to employ Indian troops in their fights against the Greeks. Darius also fought the fierce, nomadic Scythian people, whom he defeated in 513 BCE. He would likely have kept a guard of Immortals for his own protection, but cavalry would have been much more effective than heavy infantry against a highly mobile foe like the Scythians.
It is most difficult to evaluate our Greek sources when they recount battles between the Immortals and Greek armies. The ancient historians make no attempt to be unbiased in their descriptions. According to the Greeks, the Immortals and the other Persian soldiers were vain, effeminate, and not very effective compared with their Greek counterparts. If that is the case, however, it is difficult to see how the Persians defeated the Greeks in numerous battles and held on to so much land adjacent to Greek territory. It is a shame that we do not have Persian sources to balance the Greek point of view.
In any case, the story of the Persian Immortals may have been distorted over time, but it is obvious even at this distance in time and space that they were a fighting force to be reckoned with.