Woodbine - I think most of the terrorist acts in Europe have been carried out by that countries nationals. How does that get solved? Very very difficult.
Sometimes, but not always. As these 'nationals' are usually quite recent arrivals ( last 20 years), I don't think it would be too difficult to send them back.
But more urgently, we need to stop more arriving. In the last few weeks, security services foiled a plot in Dusseldorf by four people who had arrived in Mad Merkel's swarms. It won't be easy, but before they arrived in big numbers, muslim terrorism was rare in Europe. Our ancestors have spent hundreds of years keeping them out. Now we know why.
If you have any workable ideas to solve the problem, I would be interested.