Why does the Torah insert the Parshah about going to war here?
Rashi #1: It inserts it after "Ayin be'Ayin ... ", to teach us that someone who is missing a limb may not go out to war.
Rashi #2: To teach us that if Yisrael carry out justice and righteousness, they will win ytheir battles. 1
Why does the Torah add the words "Al Oyvecha"?
Rashi: One should treat enemies as enemies - and not have mercy on them, since they will not have mercy on us.
Why does the Torah write "Sus va'Rechev in the singular?
Rashi: Because in the Eyes of Hakadosh-Baruch-Hu, they are all like one horse and one rider.
Having written "Sus va'Rechev" in the singular, why does the Pasuk then state "Am Rav mimcha"?
Rashi: 'It is only to you that they appear to be a vast army, but to Hakadosh-Baruch-Hu they are like one.