Albert Barnes Commentary Jeremiah 46:2

"Of Egypt: concerning the army of Pharaoh-neco king of Egypt, which was by the river Euphrates in Carchemish, which Nebuchadrezzar king of Babylon smote in the fourth year of Jehoiakim the son of Josiah, king of Judah." — Jeremiah 46:2 (ASV)

Against ... - That is, relating to, concerning. So Jeremiah 48:1; Jeremiah 49:1; see the note at Jeremiah 46:13.

Pharaoh-necho - See the note at 2 Kings 23:29.

In - (at) Carchemish - (The Gargamis of the inscriptions, now Jerabis, on the Euphrates, about 16 miles south of Birejik.)