Why were the Persian Immortals called the Immortals?

The Immortals served as an Imperial Guard and a standing army during the Persian Empire's expansion and the Greco-Persian Wars. They were so named because the group always contained exactly 10,000 men, as each man who retired, was killed, wounded, or seriously ill would be immediately replaced by a reserve.

Explanation:

“the battle of stalingrad is not a turning point necessarily in strategic terms, because a lot more has to be done before the soviets can be certain of defeating germany,” says renowned world war ii historian richard overy. the germans had fought nearly a thousand miles across the soviet union to get to stalingrad.