Merkel has been a target since the gang assaults on women last New Year's Eve. You get to travel to these places and talk to folks first hand (I laud you, but I'm not particularly jealous because I'm not much of a traveler--just ask Mrs. 50#) and Germany is going to be an interesting case. They have welcomed folks with open arms. I was in Freiburg and Leipzig over a decade ago and I was struck by how many Turks were in he workforce. I was told it was because of the labor shortage (and that is a problem throughout
kid-starved Europe).
Here's the NPR story from yesterday on the matter. It covers both sides of the issue:
http://www.npr.org/2016/12/20/506337111/berlin-attack-sparks-debate-over-german-refugee-policy
As for stopping it, my guess is that there will be tighter immigration standards worldwide, which I don't object to. Refugees will likely stay in camps for longer stretches. But I don't think that stops the violence due to homegrown terrorism. The disaffected have so many communication and weapon options available to them that there's nothing to really curb that unless we put chips in everyone's brain. I kid, of course, but I don't see how we can stop every single incidence of terrorism. And I agree with cajun that bombing them back to the Stone Age only pushes those with mild anti-American sentiments further against us, both at home and abroad.