John Calvin Commentary Deuteronomy 20:1

"When thou goest forth to battle against thine enemies, and seest horses, and chariots, [and] a people more than thou, thou shalt not be afraid of them; for Jehovah thy God is with thee, who brought thee up out of the land of Egypt." — Deuteronomy 20:1 (ASV)

When you go out to battle. This law also, which concerns their political government, is a supplement to the First Commandment, enacting that they should carry on their wars under God's direction, and, trusting in His help, should follow Him as their leader. For they were to give this proof of their piety, so as to look to God no less in war than in peace, and not to rest their hopes of safety on anything but the invocation of His name. From this we gather that the worship of God should by no means be overlooked in civil and earthly government; for, although its direct object is to preserve mutual equity between people, religion always ought to hold the first place.

The sum, therefore, is that, amidst the very clang of arms, they must not be in such confusion as not to recognize that they are under the guardianship of God, or to lose the confidence that they will be safe in reliance on His power.

He does not, however, encourage them rashly to engage in war, but takes it for granted that there is a legitimate cause for it, because it would be a gross abuse of God’s name to seek a prosperous outcome from Him when we are engaged in anything contrary to His command. But He forbids them to fear, although the enemy should be superior in horses, in multitude, and in all their warlike array. In these words He reminds them that they would not be liable to suffer defeat despite the fact that they were not supplied with an abundance of chariots and horses. For we have lately seen that not even their kings were permitted to collect the forces in which the Gentile nations gloried. Therefore, so that the awareness of their weakness would not make them afraid, God declares that His strength would be a sufficient safeguard to them.

And without question, that passage in Psalm 20:7 is drawn from this: Some trust in chariots, and some in horses; but we will remember the name of the Lord our God. For this reason, Isaiah reproves the people because, refusing the waters of Shiloah, they long for great and rapid rivers; namely, as he elsewhere explains it, because they trust in the horsemen of Egypt (Isaiah 8:6; Isaiah 31:1). But we must observe on what their security is to be founded: namely, that the people ought to hope that the same Divine power, which their fathers had experienced when they were redeemed from Egypt, would be with them to the end.