Charles Ellicott Commentary Deuteronomy 20:3

"and shall say unto them, Hear, O Israel, ye draw nigh this day unto battle against your enemies: let not your heart faint; fear not, nor tremble, neither be ye affrighted at them;" — Deuteronomy 20:3 (ASV)

Let not your hearts faint, fear not. —In these words Isaiah strengthened Ahaz (Deuteronomy 7:4): fear not, neither be faint-hearted.

Tremble. —As in the Margin, “ make haste.” (Compare to 2 Samuel 4:4, and 2 Kings 7:15).

Be ye terrified. —A strong word. The idea is, “do not even be unnerved, much less alarmed, at the sight of them.”