About whom does the verse bewail?
Rashi: Hoshe'a [ben Elah] and the 10 tribes, who descended to Egypt for help. He sent messengers to Soa Melech Mitzrayim.
On which horses do they rely?
Rashi: On horses from Egypt; they run quickly.
Why does it say "v'Lo Sha'u Al Kedosh Yisrael"?
Rashi: They did not do like Chizkiyah did - "ba'Shem Elokei Yisrael Batach; ? va'Yimrod b'Melech Ashur" (Melachim II, 18:5, 7). Radak - Sha'u is trust, like "Yish'eh ha'Adam Al Osehu" (above, 17:7).
Malbim: Sometimes, even though someone trusts in Hashem, he seeks also natural means and does not rely on a miracle. This is not sinful for all people. However, if one knows that also the means he will receive from Hashem, and he is used to miracles, for him it is a sin. Kedosh Yisrael hints to Hashgachah Pratis and the wonders that he does for the need of Yisrael. They should have relied entirely on His Hashgachah.
After saying "v'Lo Sha'u Al Kedosh Yisrael", why does it add "v'Es Hashem Lo Darashu"?
Malbim: Sometimes, not only does one seek natural means and rely on them - he also trusts in them, and totally removes his trust from Hashem, and thinks that everything depends on the power of his hand and his actions. This is a great sin. Here it says that "they trusted in the chariots for they are many, and the cavalry for they are awesome, and do not seek Hashem (conduct of the Klal)." They totally denied Him, and trusted only in people.
