combat boots on the ground i'd suggest. the longer we wait the bigger isis will grow. they are in war since quite some time now and they get better with strategy, planning, propaganda, etc. also they have many sleepers placed all over europe, even in the us/uk and once activated they will travel down to join them to fight. we don't know the numbers probably never will, but if i had to do a guess, i'd say at least 50k if not 100k. if there was just a law in place forbidding them to travel out and back to europe, so far i have heard nothing of it and this is the big scandal for me.
I already mentioned that but yet again it's only a short term solution (and I agree that we do need an immediate short term solution now) but it still won't solve it long term.
As for travel restrictions, I'm positive there are already some but you can avoid that by flying to another country first and then flying (or driving / train / boat etc).
ISIS is not in a war, they aren't winning battles because they have superior weaponry or manpower. They're winning battles by making people so afraid of them that they'd rather abandon their cities and bases and flee than risk getting captured. Look at all the battles they've fought, the majority of the time that they've fought against even a militia or what resembles a conventional army they have lost a lot of members and sometimes even the battle itself. So what do they do to win? They go around beheading people to spread terror. At the end of the day they're nothing more than a bunch of pathetic cowards wearing masks to hide their identity and killing innocent people.
That's what makes fighting them different to a conventional army. I'm pretty damn sure if the US dropped troops and brought in air support into the region, we'd be able to wipe them out in all but a couple of days. But we won't get everyone and their entire ideology will still exist and continue to grow and THAT is what we need to destroy and that's why we need a long term plan that goes further than "keeping soldiers in those areas."
I don't mean education in the sense of "they all have university degrees" but more like what made these people give up their lives in the west (and the rest of the world for that matter) and move to the Middle East to commit these atrocities. Normal people don't just get up one morning and go "yup today is the day I go behead a whole bunch of people"
Unfortunately, ISIS already has a lot of people with university degrees. They're using their education to be even deadlier.
ISIS must be destroyed.
It's propaganda and to counter propaganda we need education. I'm sure there are other factors too but the strongest tool we have to fight terror is education.