WORLD CIVILIZATION

Detailed explanation-1: -Whether motivated by Greek pride or the spoils of imperial conquest, Alexander picked up where his father left off and marched into Persia in 334 BC, where his army of 50, 000 would be tested against the largest and best-trained fighting force in the known world.

Detailed explanation-2: -In less than a century, they conquered the Medes, Lydians, Neo-Babylonians, and eventually, the Egyptians. At its height in 500 BCE, the population of the Persian Empire was around 50 million.