Jamieson, Fausset and Brown on Jeremiah 46:2

2. Inscription of the first prophecy.

Pharaoh-necho--He, when going against Carchemish (Cercusium, near the Euphrates), encountered Josiah, king of Judah (the ally of Assyria), at Megiddo, and slew him there (2Ki 23:29; 2Ch 35:20-24); but he was four years subsequently overcome at Carchemish, by Nebuchadnezzar, as is foretold here; and lost all the territory which had been subject to the Pharaohs west of the Euphrates, and between it and the Nile. The prediction would mitigate the Jews' grief for Josiah, and show his death was not to be unavenged (2Ki 24:7). He is famed as having fitted out a fleet of discovery from the Red Sea, which doubled the Cape of Good Hope and returned to Egypt by the Mediterranean.