In 609 BC, Pharaoh Neco was heading to the Euphrates River to assist the Assyrian king against the Babylonians at Haran. Josiah confronted him at Megiddo.
After Assyrian king Ashurbanipal died in 626 BC, Assyrian cities began falling to Nabopolassar, the king of the growing Neo-Babylonian empire.
Nabopolassar captured Nineveh, Assyria's capital, in 612 BC. The Assyrian forces retreated to Haran, where the Babylonians defeated them in 609 BC.
Nebuchadnezzar II of Babylon later defeated the remaining Assyrians at Carchemish in 605 BC, the same year he first invaded Judah (see 2 Kings 24:1).