Rashi on Numbers 21:23/רש"י על במדבר כ״א:כ״ג

1לא נתן סיחון וגו׳ AND SIHON WOULD NOT SUFFER [ISRAEL TO PASS THROUGH HIS BOUNDARY] — For all the Canaanite kings paid him tribute, because he protected them that no hostile forces should pass through his land against them. As soon as Israel said to him, “Let me pass through they land”, he answered them: “The whole purpose of my dwelling here is to protect them against you, and yet you speak thus!” (Midrash Tanchuma, Chukat 23).

2ויצא לקראת ישראל AND HE WENT OUT AGAINST ISRAEL — Even if Heshbon had been full of people as puny as gnats, no human being could have captured it, and if Sihon had been (resided) in a weak village no human being could have overcome him. How much the more was he invincible whilst he dwelt in Heshbon. Therefore the Holy One, blessed be He, said: Why should I trouble My children to all this extent, to besiege each city? He therefore put it into the hearts of all the men of war to go forth from the cities, and they assembled all of them in one spot, and there they fell in battle; from there Israel proceeded to the cities and there were none to resist them, for there were only women and children there (Midrash Tanchuma, Chukat 23).