פירוש על דברים 1:30

ילחם לכם means, [HE WILL FIGHT] ON YOUR BEHALF.

He will wage war for you. Hashem’s intent was not that the Canaanites should be smitten by plague before they arrived to wage war, but that He would help the Jews while they were fighting against them.

As in everything that He did with you. With the Egyptians — Hashem did not destroy them beforehand but waged war against them at the Red Sea. This was His Will and His Honor, and similarly in the war against the Canaanites.