What did we hear?
Rashi: We heard consolations from the Nevi'im - so we saw!
Radak: We heard from Nevi'im about the fall of Gog and Magog.
Malbim: This is Ma'amar Musgar (parenthetical comment) - just like we heard about this wind, so we saw in Ir Hashem, the same storm wind that was at sea.
What did we see?
Rashi: We saw like the consolations that we heard from the Nevi'im.
Malbim: The wind that broke the boats of Tarshish, the same wind established Yerushalayim that the enemies will not destroy it.
Why does it say "Hashem Tzevakos"?
Radak: He is Master of legions above and below, and He brought these legions to Yerushalayim, and He will topple them there.
Why does it say "b'Ir Elokeinu"?
Radak: He is Elokeinu and our Judge against awesome nations.
Why does it say "Selah"?
Malbim: This completes the matter.