Why Persian “Immortals” Never Declined

In the 5th century BCE, the Achaemenid Empire maintained an elite infantry unit known as the Persian "Immortals." They were not called that because they could not die, but because their number was kept at exactly 10,000 soldiers at all times. Whenever a man was killed, wounded, or fell ill, another soldier immediately replaced him. This constant replacement created the illusion that the unit never suffered casualties, which is why Greek historian Herodotus described them as the "Immortals."

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