What is the Pasuk referring to when it writes "Acheil Teis Pachd'cha ... Tachas Kol ha'Shamayim"?
Rashi and Targum Yonasan: It refers to the fact that, 1 during the battle against Og, the sun (and the moon - Targum Yonasan) stood still 2 (and stopped singing Shirah - for thirty-six hours - Targum Yonasan [to allow the army to defeat the enemy]) - something of which the whole world was aware. 3
Ramban: It refers to all the nations who would become terrified of Yisrael 4 - even when the men of Cana'an went to fight Yisrael, they went 'with melted hearts', only Hashem hardened their hearts and gave them to believe that it was better to die by the sword as free men than to become slaves to Yisrael.
Oznayim la'Torah: The mere victory over the mighty Sichon and Og, whom the nations had hired to protect them against enemy attacks, revived the fear of Yisrael that the nations had felt after K'ri'as Yam-Suf, which had dissipated when Yisrael wandered aimlessly in the desert whilst bypassing Edom, Mo'av and Amon without attacking them.
See Sifsei Chachamim which, citing Ta'anis, 20a, Darshens this via a Gezeirah Shavah from ?Acheil Gadelcha, in Yehoshua 3:7.
See also Ba'al ha'Turim.
Oznayim la'Torah: And the miracle would mpress upon them that the defeat of Sichon (and subsequently of Og), was not by chance, but the result of Hashem's participation.
Ramban: Otherwise, bearing in mind that Moshe will fight only two nations, why would Hashem place the fear of Moshe on all the nations? The Pasuk is therefore coming to reassure Yisrael and Yehoshua that the nations will fear them (below, 3:21). And this is the reason for the command "Begin to inherit his land" (Pasuk 31).
