What many nations did He strike?
Radak: Yisrael were few; they leave harsh labor and slavery, and these are many and awesome in their land (Kena'an). Yisrael struck them, and took their land. Who did this? Hashem's desire did it.
What is the significance of striking many nations and killing awesome kings?
Malbim: Also this was miraculous. I did not need this to recognize Hashem's strength; rather, He sent signs to Sichon and Og, who plastered over their eyes to avoid seeing Him from the creation and His conduct. He needed to change nature for them 1 [to see Him].
I.e. they saw miracles in the war before they died. Similarly, "v'Yad'u Egypt Ki Ani Hashem" (Shemos 14:18), even though the better Egyptians drowned quickly (they sank like rocks or lead. - PF)
