Why did he say "b'Ad Amenu"?
Radak: Lest they be killed by the sword or taken captive.
Why did he say "b'Ad Arei Elokeinu"?
Radak: Lest the enemy conquer them and settle in them, if they overpower us and defeat us. Then they will not be Arei Elokeinu, rather, Arei Elohim Acherim!
Malbim #1: If the enemy overpowers, they will destroy the cities of Eretz Yisrael.
Malbim #2: In addition to natural preparation ("Chazak v'Nischazek"), we must prepare intent l'Shem Hashem - b'Ad [Arei] Elokeinu.
Why did he say "Hashem will do what is good in His eyes"?
Radak: We will strengthen ourselves in war as much as we can, and He will do what is good in His eyes, for salvation is His.
Malbim: He will help according to the proper preparation (refer to 10:12:2:3).
