One man of you puts to flight a thousand, since it is the LORD your God who fights for you, just as he promised you.
Moses speaks here of what they shall be able to do if they remain faithful to the Lord, not what they had already done; one man will chase a thousand because the Lord Himself fights for them. Samson proved this in fact. When God is your warrior, no enemy multitude can stand.
AI summary
God pledges what He has already given as proof of what remains to be fulfilled. But His promise depends entirely on their keeping themselves clear of the Canaanites and their idolatry. Steadfast adherence to God's law will bring brilliant victories; any other path breaks the condition.
AI summary
Commenting on Joshua 23:5-11
One man of you shall chase a thousand - Do not remain inactive on the supposition that you must be much more numerous before you can drive out your enemies, for it is the Lord that shall drive out nations great and strong; and under his direction and influence one of you shall chase a thousand.