Why is there the prefix Hei in "ha'Tzevi"?
Radak: The Hei is of Keri'ah (he calls to them), like "ha'Kahal Chukah Achas Lachem" (Bamidbar 15:15), "ha'Dor Atem Re'u Devar Hashem" (Yirmeyah 2:31) and similar verses.
Why does it say "ha'Tzevi Yisrael"?
Radak: Tzevi is desire, like "Ki Sihyu Atem Eretz Chefetz" (Mal'achi 3:12). The word to which ha'Tzevi is Samuch is missing; it should say ha'Tzevi Tzevi Yisrael. This is like "ha'Aron ha'Bris", "ha'Am ha'Milchamah" (Yehoshua 3:14, 8:11). I.e. you, the desired land, how is it that corpses fell on your high places?! How did Giborim fall?
Radak citing Targum Yonasan: Yisrael standing on their fortified places were cast down as corpses. Ha'Tzevi is like Nitzav Netzavim.
Malbim: Even though Yisrael needed to learn archery, they should have been victorious even naturally, for two reasons. (a) "Al Bamosecha Chalal" - Yisrael were on the mountain. Even the weak above should defeat the strong below! (b) "Eich Nafelu Giborim" - they were strong. Rather, we must say that Yisrael were defeated due to Hashgachah - "Ki Tzuram Macharam va'Shem Hisgiram" (Devarim 32:30).
