Pharaoh-nechoh: i.e. Nechoh II, the sixth king of the twentysixth dynasty. His father was a tributary to Assyria, but had secured independence for Egypt.
the king of Assyria: i.e. the king of Babylon, who had just conquered Nineveh, the rival capital.
went against him. His motive not known.
he = the king of Egypt.
him = Josiah.
Megiddo. Southern margin of the plain of Esdraelon, celebrated for Syria's defeat by Barak (Jdg_5:19).
seen. Figure of speech Tapeinosis, to emphasize the fact that he did much more than "see" him. Compare 2Ki_14:8 and 2Ch_35:21, 2Ch_35:22.