For me the most terrifying thing is that these disturbed, separated from the rest of humanity, brainwashed and quite often simple minded people who somehow manage to suppress the most basic of feelings humans share (compassion, mercy, self preservation etc) do not have the appearance of monsters, and neither do they come with a tag on them so the rest of us can identify them and do something about the threat they represent. They walk the streets like you and me, some of them have families, some of them have everyday jobs. I presume, anyway, as I am in no position to know for sure. At least in the case of Charlie Hebdo and the London attack on Lee Rigby this was the case.
In the case of ex colonial powers people from different creeds, religions or colour have been residing in Europe for decades with little or no trouble at all, at least to my knowledge. I believe the problem has been created relatively recently (in the last 15-20 years) due to the catastrophic foreign policies the people across the pond have pursued and somehow managed to get some European countries involved in too. It gave the minority of Muslims who want to see Europe perish an excuse. It has worked admirably, and unfortunately to Europe's detriment. I suspect there will be very few people who will refute this is the case. I do not doubt that amongst the countless thousands of people coming over to Europe in the last year or two there have been a (hopefully small) number of radicals who are hoping to create havoc by killing, maiming and destroying. cannot help but think that if this was a generic problem, there would have been attacks in Spain, Greece, Italy way before the refugees reached London, Paris, Berlin. The answer as to why this is not the case lays, I believe, in what I mentioned earlier about foreign policies and the plundering of foreign lands, stripping them of oil, minerals etc.
Leaving the political aspect of it out, the most important question, and a very hard one at that, is what do we do now? How do we respond to this? Escalating the bombardments, sending foot soldiers to foreign lands again is, I feel, not only counter productive, but downright stupid. It will almost certainly play in the hands of these maniacs who will see this as a self fulfilled prophecy. They will be able to say that the white man is going over to their lands killing their women and children and the same old stuff that has brought them new recruits and made them a force to be reckoned.
Close the borders all you like. It is the second and third generation people who are here either from the colonies or because of the wars we brought to their homelands that have an idealized view of their holy war. It is the grandchildren of those who came from the subcontinent that have never lived in a hut with no running water that may turn against the west. They are the most vulnerable to predatory mullahs preaching hate in European cities. It is the children who saw the death of their parents/relatives from a drone strike that want revenge.
So, if not bombs, what is the answer? Education, jobs, communication, if you ask me. Obviously for a large number of people this is too late. They are way too radicalised/disenfranchised/angry to see reason. For them a more on hands approach is necessary. For the rest of the population though there is plenty of time. The least contact you have with something/someone different to you, the easier it is to demonise it, the easier it becomes to blame it for things others tell you they are responsible for, the easier it becomes to see them as 'fair game'.
It is a tragic situation we are faced with. Tragic and extremely hard to deal with...
P.S: Pine Marten, for what it's worth, you just topped my list of people I have high esteem for.