This is something I thought about a few weeks ago and it was recently recalled to mind after watching the move Legion (not that good).
I have seen the argument that some sects of Christianity raise the Devil to pseudo-divinity in the name of dualism. The Devil is the opposite of God, seeking to undo all that God has done, and some raise his level of power (perhaps without meaning to do so) to the level of God Himself. In such a system, God must actively fend off the plans of Satan in order to maintain control of creation. This is poppycock.
From the human perspective, God, the angels, Satan, and the demons are all vastly superior to us in power. From the human perspective, like that of an ant, the difference between an elephant and a brontosaurus is irrelevant; both are orders of magnitude beyond our comprehension and either could snuff us out in an instance. But rising above the human perspective to the spiritual perspective, God is as far beyond Satan as Satan is beyond mankind.
We often speak of the incident in the Garden of Eden as Original Sin. Certainly it is so for humanity, but it is not the first instance of sin. To my knowledge, the Church teaches that the first rejection of the will of the Creator by the created was perpetrated by Satan and culminated in the war of the heavens when Satan and his demons were thrust into hell. We are led to believe that this primal sin was one of hubris, and pride has forevermore been listed as chief among the Seven Deadly Sins (and, in my own opinion, all sin must necessarily have pride at its root). Satan, no doubt great among the angels, did more than recognize the greatness he had been given; he put his will before God's and was cast from heaven.
The war between God and Satan is not a war, for to be a war there must be some threat and Satan has as much hope against God as an ant does against the sun. A better metaphor is that Satan is a petulant child who thought he was special and was laid low. Like a child, he cannot struggle with any efficacy against his Father, and so he exerts his will indirectly: by hurting us. Satan's only weapon against God is God's love for others. God desires that all men should be saved and so Satan tempts men from salvation to damnation, like a child who breaks his Father's prized possessions.
The great war of the Apocalypse, as envisioned by fundamentalist Christians, may very well one day come. But far from being an epic struggle between God and Satan, it will only be a struggle between Satan and us. God's victory has never been in question; even Satan has known that since his beginning. Rather, the question is the extent of God's victory. How many men will be claimed by Satan before he is sequestered for all eternity in the pit of fire?