Religion

Since the Fifties, Germany accepted a lot of Turkish citizens to come in and do labor they didn't want to do. Like we did with Mexicans and later, Central and South America. The German economy depends upon their work.

And most of these Turkish visitors were given the equivalent of Green Cards. And they stayed. And sent for their families. And looked down upon by Germans. They never became German. And never went home.

There is a significant demographic of these people in Germany to this day. And they're Muslim. And with the situation in the MidEast, it's easy to see how the younger folks could become radicalized.