Albert Barnes Commentary Zechariah 12:4

"In that day, saith Jehovah, I will smite every horse with terror, and his rider with madness; and I will open mine eyes upon the house of Judah, and will smite every horse of the peoples with blindness." — Zechariah 12:4 (ASV)

In that day, says the Lord, I will smite every horse with astonishment, stupefying - Zechariah revives the words emphasized by Moses to express the stupefaction at their troubles, which God would heap upon His people if they persistently rebelled against Him. Each expresses the intensity of the visitation.

The horse and his rider (Deuteronomy 27:28) had, through Moses’ song at the Red Sea, become the emblem of worldly power, overthrown. That song opens: I will sing to the Lord; for He has triumphed gloriously: the horse and his rider He has cast into the sea (Exodus 15:1). The frightened cavalry throws into confusion the ranks of which it was the boast and strength.

And on the house of Judah I will open My eyes - In pity and love and guidance, as the Psalmist says, I will counsel, with My eye upon you (Psalms 32:8), in contrast with the blindness with which God would smite the powers arrayed against them.