David continues telling us about his victory over his enemies, and David had to overcome several, there was Saul his adversary, Goliath the fighter in the field and then his own son Absolom who tried to stage a coup against David, and David 

Psa 18:40  You placed my foot on their necks. I have destroyed all who hated me.

Psa 18:41  They called for help, but no one came to their rescue. They even cried to the LORD, but he refused to answer.

We learn there that God was with David, and not David’s foes, carefully noting again here that David declares these victories as God’s, not his….in other words, these wins came at the hands of God, not David’s , as we note in Verse 40 where he says “Lord, you placed my foot on their necks” or in 41 “they called for help but You did not answer” 

So without God, none of this would have happened as David tells us in the next verse:

Psa 18:43  You gave me victory over my accusers. You appointed me ruler over nations; people I don’t even know now serve me. . 

So David takes the throne because God allowed it.  He is the head of all of Israel.  Quite the contrast to today’s politicians, who like to take their own credit, a politician’s favorite word is I, they can’t say enough good things about themselves, but we compare that to what David says, and David says, not i but God.  God made me the ruler of nations, God did that, and further, I couldn’t have done that without God.  Big difference between David and today’s heads of state.