Final answer:
Cyrus the Great expanded the Persian Empire to become the largest empire of its time, conquering various territories including the Neo-Babylonians.
Step-by-step explanation:
The leader of the Persian Empire who extended it to become the largest empire the world had yet seen was Cyrus the Great. During his reign, Cyrus the Great led a series of campaigns that vastly expanded the territory of the empire. He conquered the Neo-Babylonians, incorporating their territories which included Mesopotamia and Egypt into the Persian Empire. His efforts and military campaigns successfully created the largest empire the world had seen during that era, covering regions from the Balkans in the west to the Indus River valley in the east.