How can it be quiet - This is the answer of the Sword. I am the officer of God's judgments, and he has given me a commission against Ashkelon, and against the sea shore; all the coast where the Philistines have their territories. The measure of their iniquities is full; and these God hath appointed this sword to ravage. The Philistines were ever the implacable enemies of the Jews, and the basest and worst of all idolaters. On these accounts the sword of the Lord had its commission against them; and it did its office most fearfully and effectually by the hand of the Chaldeans.

How can it be (h) quiet, seeing the LORD hath given it a charge against Ashkelon, and against the sea shore? there hath he appointed it.
(h) Meaning, that it is not profitable that the wicked should by any means escape or hinder the Lord when he will take vengeance.

How can it be quiet,.... There is no reason to believe it will, nor can it be expected that it should; to stop it is impossible, and to request that it might be stopped is in vain:
seeing the Lord hath given it a charge against Ashkelon, and against the seashore? for it had a commission from the Lord to destroy the inhabitants of Ashkelon, and other places, which lay still more towards the sea, as Joppa and Jamne; and indeed all Palestine lay on the coast of the Mediterranean sea:
there hath he appointed it; by an irreversible decree of his, in righteousness to punish the inhabitants of these places for their sins.

Jeremiah, from addressing the sword in the second person, turns to his hearers and speaks of it in the third person.
Lord . . . given it a charge-- (Ezekiel 14:17).
the sea-shore--the strip of land between the mountains and Mediterranean, held by the Philistines: "their valley" (see on Jeremiah 47:5).
there hath he appointed it-- (Micah 6:9). There hath He ordered it to rage.
It had taken part with the Chaldeans against Judea (2-Kings 24:2). Fulfilled by Nebuchadnezzar five years after the destruction of Jerusalem, when also he attacked Egypt (Jeremiah 43:8-13) and Ammon (Jeremiah 49:1-6). [JOSEPHUS, Antiquities, 10:9,7]. Jeremiah in this prophecy uses that of Isaiah. 15:1-16:14, amplifying and adapting it to his purpose under inspiration, at the same time confirming its divine authority. Isaiah, however, in his prophecy refers to the devastation of Moab by the Assyrian king, Shalmaneser; Jeremiah refers to that by Nebuchadnezzar.

How - God lets the prophet know that he had given this sword its commission, and therefore it could not stop 'till Ashkelon and the people on the sea - shore were destroyed by it.