Under king Darius I the Persian Empire became the greatest empire the world had ever seen. The empire came to an end in 334 BC when Alexander the Great invaded the Middle East.

Alexander's invasion of northwestern India weakened the defenses of that area and perpared the way for Chandragupta Maurya to add the Indus region to his domain. His grandson, Asoka, established the first unfied empire over most of the Indian peninsula.