“And they will know that I am Yahweh when I have executed judgments in her and am sanctified in her. For I will send pestilence into her, and blood in her streets, and the wounded will fall in the midst of her, with the sword at her from every side, and they will know that I am Yahweh.”
Yahweh's servant Nebuchadnezzar, and his army, will devastate Zidon revealing that Yahweh is different from all gods and fulfils His purposes as the all-powerful One (is ‘sanctified in her' - compare Ezekiel 38:16; Ezekiel 36:23). They will suffer pestilence and slaughter, regular accompaniments to warfare, and will perish under the sword. The sword may be that of Yahweh (Ezekiel 21:5) or simply represent the swords of Nebuchadnezzar's army.
Thus will they too realise Who and What Yahweh is, repeated twice at beginning and end for emphasis.