Why does David need a strong army if God is on his side?


Day after day men came to help David, until he had a great army, like the army of God. 1 Chronicles 12:22
            The army that met David came from many tribes within Israel.  He had the ability to unite Israel starting with the armies.  As a result, the people followed.
            So does a nation who follows God need an army if God promises to protect them.  No, but we see God using those armies to show his power.  We also see times when the army does nothing but God splits a sea or wipes out an enemy through confusion or plague. 
            The army is a visual representation of the power of God on earth but we know they could not be victorious without God on their side.