Even if the Christians and the Muslims did unite to defeat this common foe, they would once again be pitted against each other afterwards. The way the human mind is engineered there is not absolute "peace", just a constant battle for superiority. The only thing that keeps us from acting like the barbarians is that we like our current way of life and by creating the passive unspoken treaties with our neighbors we are, in effect, only buying time for when we will defeat them as soon as we can do without them. Pause and think for a moment, what has happened, what is happening, what will happen until the passing of man from the earth is the constant struggle for absolute power. Of course, most of us never live to see the manifestation of our sub-consiousness.