Why did Hashem tell him not to come upon them?
Radak: It is not yet the time for the salvation.
Radak (24, citing Shochar Tov 27): You have no permission to strike them, even if they are close to you, until you see the tops of the trees moving.
Malbim: It says in Divrei ha'Yamim I, 14:14 "do not come upon them from the back", i.e. like the first time. Rather, go around, so you will be on one side and the trees on the other side. Refer to 5:24:3:4.
Malbim: Sha'ul lost his kingship because he did not wait for Shmuel. David is being tested - he must wait for Hashem, and not rely on his own strength!
What is "mi'Mul Becha'im"?
Rashi citing Targum Yonasan: It is opposite the trees. Radak - Chazal call these Tusim (berry bushes). The same applies to "Emek ha'Bacha" (Tehilim 84:7).
