Albert Barnes Commentary Jeremiah 47:1

"The word of Jehovah that came to Jeremiah the prophet concerning the Philistines, before that Pharaoh smote Gaza." — Jeremiah 47:1 (ASV)

that Pharaoh - Pharaoh-Necho, though defeated at Carchemish, was probably able to seize Gaza upon his retreat, when obviously the possession of so strong a fortress would be most useful to him to prevent the entrance of the victorious Chaldeans into Egypt.