Albert Barnes Commentary 2 Kings 23:29

"In his days Pharaoh-necoh king of Egypt went up against the king of Assyria to the river Euphrates: and king Josiah went against him; and [Pharaoh-necoh] slew him at Megiddo, when he had seen him." — 2 Kings 23:29 (ASV)

Pharaoh-Nechoh - This king is well known to us from both secular historians and Egyptian monuments. He succeeded his father Psammetichus (Psamatik) in 610 B.C. and was king of Egypt for 16 years. He was an enlightened and enterprising monarch. The great expedition mentioned here was an attempt to detach the important tract of country extending from Egypt to the Euphrates at Carchemish from the newly-formed Babylonian empire. Likely counting on the friendship or neutrality of most local powers, the Egyptian monarch prepared for two years before setting out on his march. He probably followed the usual coastal route through Philistia and Sharon, intending to cross from there by Megiddo into the Jezreel (Esdraelon) plain.

The king of Assyria - This expression does not imply that Nineveh had not yet fallen. Accustomed to Assyrian monarchs who held their courts alternately at Nineveh and Babylon (2 Kings 19:36; 2 Chronicles 33:11), the Jews at first regarded the change as merely dynastic. They therefore transferred to the new king, Nabopolassar, the title they were accustomed to giving their former overlords. Later, when Nebuchadnezzar invaded their country, they found that he called himself “King of Babylon,” not “King of Assyria,” and from then on, that title came into use. However, the annalist who wrote about Josiah’s life immediately after his death—an account the author of Kings copied—understandably used the more familiar, though less correct, title.

Josiah went against him - Josiah likely felt duty-bound to oppose the march of a hostile force through his territory to attack his overlord. For further details, see the parallel account in Chronicles. Regarding Megiddo, see Joshua 12:21.