Deuteronomy 20:1
When thou goest out to battle against thine enemies, and seest horses, and chariots, and a people more than thou, be not afraid of them: for the LORD thy God is with thee, which brought thee up out of the land of Egypt. When thou goest out to battle against thine enemies, and seest horses, and chariots, and a people more than thou, be not afraid of them: for the LORD thy God is with thee, which brought thee up out of the land of Egypt.
Deuteronomy 20:1 stands within Moses' recounting of Israel's experiences and God's guidance as one constant appeal to serve the Lord and turn away from the sins that wrecked the other nations. In that setting the precepts of the law are unfolded together with God's gracious promises, showing that Israel's greatness and continuance lay only in serving Him. When Israel went out to battle and saw horses and chariots and a people more numerous and better equipped than themselves, they were not to be afraid, because the Lord their God was with them. That divine presence itself was the reason for courage against a larger enemy force. The Lord's act of bringing Israel up out of the land of Egypt was the ground of this assurance not to fear the stronger enemy.
The Lord promised in this way to fight the battles for His people in the campaign against the inhabitants of Canaan if they would be faithful to Him. That conquest began with Jericho as the firstfruits of the campaign. There Israel had to manifest faith in God and learn dependence upon Him for success. There also they had to recognize His first claim upon all that was gained in battle.