Midrasch zu Schoftim 6:16

For in their anger they slew a man (Gen. 49:6). Did they slay only one man? Is it not written: And they came upon the city unawares, and they slew all the males (ibid. 34:25)? The fact is that they were all considered by the Holy One, blessed be He, and by them as one man, as Scripture states: And thou shalt smite the Midianites as one man (Judg. 6:16). Similarly, it states: The horse and its rider hath He thrown into the sea (Exod. 15:1).

And you see horse and chariot (Deuteronomy 20:1): And it is stated (Exodus 15:19), "For when the horse of Pharaoh." And why did it not state, "horses and riders?" Rather [it is] because in front of the Holy One, blessed be He, they are only considered like one horse. "A people more numerous than you." The Holy One, blessed be He, said, "They are numerous in front of you. But in front of Me, they are only only considered like one man." And from where [do we know this]? As so did the Holy One, blessed be He, say to Gidon (Judges 6:16), "and you shall smite Midian like one man." And from where [do we know] that they are numerous? As it is stated (Isaiah 17:12), "Ah, the roar of many peoples that roar as roars the sea." And for each one of you, a hundred [to] five, but you are victorious, as it is stated (Leviticus 26:8), "And five of you shall give chase to a hundred." Therefore it is written, "a people more numerous than you." "Do not be afraid of them." Your forefathers were not afraid of them and they were victorious. What is written about our father, Avraham? "At night, he and his servants deployed against them and defeated them" (Genesis 14:16). And he was not afraid of them, as I told him (Genesis 15:1), "Fear not, Avram, I am a shield to you; your reward shall be very great." And I made him a shield, as it is stated, "I am a shield to you." And in the same way that I did [this] for him, so [too] will I do it for you. As it is stated (Isaiah 14:24), "As I have designed, so shall it happen; what I have planned, that shall come to pass." What is [the meaning of] "As I have designed?" As I have done for Avraham, so shall I do for you.