For a moment, David trusted in the size of his army rather than in the Lord. For that the Lord sent a prophet to David for him to choose one of three options as punishment. He chose falling into the hand of the Lord, and this is what happened,

So the LORD sent a plague upon Israel from the morning till the appointed time. From Dan to Beersheba seventy thousand men of the people died (2 Samuel 24.15). 

Rapidly the Lord depleted David’s trusted army, and he knew that he had done wrong. Since the Lord did this, we know that He put meaning into the numbers of the dead, which in the New King James Version stands at 70,000!

The Lord got His message across quickly that David, and by extension all men, should trust only in the Lord, for He can decimate a large army quickly. The Lord loves the number 7 and to get His message across to David, the Lord multiplied that number 10,000 times, shouting to David that his army did not stand a chance when he neglected to think of the Lord, which was a rarity in the life of David.