He rose into revolt against the Assyrian Empire (Which had ruled Babylon for the last 200 years) in 625 BC after the last really powerful Assyrian king, Assur-bani-pal died in 627 BC.
The weakened Assyrians couldn't resist his power and that of the Medes who sacked the Assyrian capital Nineveh in 612 BC. Nabopolassar was left in control of Nineveh and destroyed the remnants of the Assyrian Empire in 609 BC.
Nabopolassar waged war against Egypt from 610 BC till 605 BC. In 605 BC his son Nebuchadnezzar won the Battle of Carchemish shortly before Nabopolassar died. Nebuchadnezzar succeeded him to the throne of Babylon.