Horses and chariots are not particularly suitable for fighting in the hilly areas of Israel, though they were effective on the plains.1 By hamstringing the horses so that he has a limited number to use, David testifies to his trust in the Lord rather than military might (see also Deuteronomy 17:16).2
4 And David took from him 1,700 horsemen, and 20,000 foot soldiers. And David hamstrung all the chariot horses but left enough for 100 chariots.