Who was Balak?
Rashi (in Pasuk 4): He was one of the five princes of Midyan, and when Sichon was killed, Mo'av appointed him king (to defend them against Yisrael).
Ramban #1 (in Pasuk 4) and Seforno: He was a renowned warrior of that time. 1
Moshav Zekenim (citing R. Yehudah ha'Chasid): He was one of Sichon''s nobles; then, he saw what Yisrael did to the Emori. This resolves Shoftim 11:25, which implies that Balak did not fight Yisrael, with Yehoshua 24:9, which says that he fought Yisrael! He fought when he was with Sichon; he did not fight after becoming Melech Mo'av 2
Seforno: As is evident from a statement by Yiftach in Shoftim, 11:25. Ramban: And the Torah is telling us that, in spite of that, Mo'av was afraid.
Moshav Zekenim: R. Moshe rejected this. The latter Pasuk calls him "Melech Mo'av". This implies that he fought when he was king of Mo'av! Also, the Pasuk says that he called to Bil'am - it was at the time that he called to Bil'am (after he became king)! This is not difficult, because the Pasukin Yehoshua is after he became Melech Mo'av.
Why does the Torah write here "va'Yar Balak", whereas in connection with Yisro, it wrote "va'Yishma Yisro"?
Oznayim la'Torah #1: Because, whereas with Yisro it was a matter of Emunah - he believed that the events described there were orchestrated by Hashem - and the Lashon 'va'Yishma' is appropriate. Balak simply saw what Yisrael had done to the Emori.
Oznayim la'Torah: Whereas Balak actually saw what Yisrael had done to the Emori - since it happened within seeing distance from his land, Yisro, who lived far from the events described there, only heard about them.
