What is "b'Tzeischa [Lifnei Amecha]"?
Radak: When Yisrael used to defeat the enemy, it was as if Hashem went out in front of them. Also Aron Bris Hashem went in front of them. It mentions the victory in the Midbar against Sichon and Og 1 . Just like Hashem went in front of them then and fought their wars, so may He do so now!
Malbim: This explains when His enemies were scattered and He settled individuals in houses. It was when You went in front of Your nation with David's army, to fight their wars.
They are not mentioned explicitly. Perhaps "vi'Yshimon" hints to them. Shiras ha'Be'er ends "Al Pnei ha'Yshimon" (Bamidbar 21:20), and right after it discusses the request to pass through Eretz Sichon and the war that occurred. (PF)
Why does it say "b'Tzadecha vi'Yshimon"?
Rashi: When You stepped with Your step.
Radak: In the Midbar, a place of Shemamah (desolation) - "Kol ha'Midbar ha'Gadol
Why does it say "Selah"?
Rashi: There You showed that this is always Your way - for every affliction, redemption.
Malbim: This completes the matter.
