Yes, it’s true that David had a great military. Yet, what does the Bible teach? Here’s Proverbs 21:31 “The horse is prepared against the day of battle: but safety is of the LORD.”
Here’s a word from David himself. Psalm 118:8 “It is better to trust in the LORD than to put confidence in man. (9) It is better to trust in the LORD than to put confidence in princes. (10) All nations compassed me about: but in the name of the LORD will I destroy them. (11) They compassed me about; yea, they compassed me about: but in the name of the LORD I will destroy them. (12) They compassed me about like bees; they are quenched as the fire of thorns: for in the name of the LORD I will destroy them. (13) Thou hast thrust sore at me that I might fall: but the LORD helped me. (14) The LORD is my strength and song, and is become my salvation. (15) The voice of rejoicing and salvation is in the tabernacles of the righteous: the right hand of the LORD doeth valiantly.”
Far too many nations have fought under their own banner so-to-speak. Far too many nations have greater confidence in themselves and in their military prowess than in the LORD. And so; men who win on the battlefield will often honor themselves.
Now look at II Samuel 8:14. (14) “And he put garrisons in Edom; throughout all Edom put he garrisons, and all they of Edom became David's servants. And the LORD preserved David whithersoever he went.”
Of all the teachings on the matter of war in the Bible, this might be the most neglected.
“Any nation that goes to war and neglects to pray is a nation destined for defeat.” You see, the LORD does preserve His own because He’s the One Who deserves the glory – even in the victories of war.