Any discussion of Islamist terrorism in Europe and the refugee crisis has to be placed in a broader historical context. One way to approach this is to think about the Mediterranean Sea, which was central to the Roman Empire.
The Romans occupied both shores of the Mediterranean and created a single integrated political and economic system around it. As the Roman Empire declined, the system fractured. The general outcome was that Christianity was prevalent on the northern shore of the Mediterranean, and Islam became dominant on the southern shore.