What was the beginning of the singing?
Malbim: This is like it says "Meshorerim la'Shem u'Mehalelim" (21).
How did Hashem set an ambush?
'Rashi': He arranged people to destroy them. This is like it says about Aviyah "v'Yarav'am Hesev Es ha'Ma'arav." So Hashem set them against each other - Mo'av helped Amon, and both of them fought Bnei Se'ir.
Malbim: Mo'av, Amon and Se'ir hated each other, just they united against Yisrael. Refer to 20:22:3:1.
Why does it say "Al Bnei Amon Mo'av v'Har Se'ir"?
Malbim: Hashem put in the heart of Mo'av to plan to destroy Amon and Se'ir, its neighbors. It covertly joined with Se'ir to destroy Amon. The ambush was against Amon; the ambushers were Mo'av and Anshei Se'ir. Refer to 20:23:1:2.
Who was stricken?
Malbim: Amon.
