יְהוָ֤ה אֱלֹֽהֵיכֶם֙ הַהֹלֵ֣ךְ לִפְנֵיכֶ֔ם ה֖וּא יִלָּחֵ֣ם לָכֶ֑ם כְּ֠כֹל אֲשֶׁ֨ר עָשָׂ֧ה אִתְּכֶ֛ם בְּמִצְרַ֖יִם לְעֵינֵיכֶֽם׃
The LORD your God who goeth before you, He shall fight for you, according to all that He did for you in Egypt before your eyes;
Haamek Davar on Deuteronomy
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.
Haamek Davar on Deuteronomy
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.