Why does it say "Tiz'ak Cheshbon v'El'ale"?
Radak: They are cities of Mo'av, and also Yahatz, which is "Yahtzah" (Bamidbar 21:23).
Malbim: These cities were the pride and glory of Mo'av. Only after they were destroyed, they began to scream 1 .
What is the significance of "Ad Yahatz"?
Malbim: The destruction reached the essence of the land (not merely the border).
What is the meaning of "Chalutzei Mo'av Yari'u"?
Rashi: The armed people of Mo'av will shout to greet war.
Radak: All the armed soldiers of Mo'av, when they saw the nation crying, also they will shout, crying and wailing with them, and not put their hearts to fight the enemy.
What is the meaning of "Nafsho Yar'ah Lo"?
Rashi: The Nefesh of Mo'av totters, like one who mourns over himself.
Radak: "Yar'ah" is like Teru'ah, even though it is from a different root. Nefesh Mo'av will be bad due to itself, i.e. the evil that came to it.
Malbim: It was not Teru'ah of vengeance and strength, rather, of breakage. It is as if they had Nevu'ah that they will perish.