Albert Barnes Commentary Amos 1:7

"but I will send a fire on the wall of Gaza, and it shall devour the palaces thereof." — Amos 1:7 (ASV)

But - Literally, “and.” As Gaza had done, so God would also do: I will send a fire upon Gaza. The sentence on Gaza stands out, probably because it was first in power and in sin. It was the merchant-city of the five; the caravans departed from it or passed through it, and so this sale of the Jewish captives was ultimately accomplished through them. First in sin, first in punishment. Gaza was strong by nature and by human construction.

“The access to it also,” Arrian notices, “lay through deep sand.” We do not hear of its being taken, except in Israel’s earliest days under the special protection of God (Judges 1:1–2), (Judges 1:18), or by great conquerors. All Philistia, probably, submitted to David; we do not hear of any specific conquest of its towns (2 Samuel 8:1). Its siege cost Alexander two months, with all the aid from the siege engines he had used to take Tyre, and the experience he had gained there. The Egyptian accounts state that when besieged by Thutmose III, it capitulated. From that time on, it had submitted neither to Egypt nor Assyria.

Yet Amos declared absolutely that Gaza would be destroyed by fire, and it was so. Sennacherib first, then, after Jeremiah had again foretold the destruction of Gaza, Ashkelon, and the Philistines, Pharaoh Necho smote Gaza (Jeremiah 47:1). Yet who, with human foresight alone, would dare to predict the destruction of such a strong city?