>1 john 5:19 We know that we are children of God, and that the whole world is under the control of the evil one.
>Ephesians 6:12 For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places.
He and the fallen angels are in control of the world, however if you look into cases of demon possessions solved by prayer, or exorcism/deliverance, you'll see that God holds the final power. The devil is only a counterfeit of what God is, an magic trick to someone who allows magic.
The devil is only strong to those who are fooled or allow evil to seep in through sin. I think the presence of the Devil is here to test good, and it sucks that he has such an ancient and resilient presence on earth.