There are not set "real ways" to stop global terrorism, and the article you link is full of mistruths and conspiracies. Implying current links between Saudi Arabia and IS is unresearched nonsense, especially considering the two have strictly different ideologies and interpretations of Islamic law. The fact is, despite having two decades to develop a play book, the us response to terror remains incredibly static and inflexible and there is no certainty what will work or what won't. The Obama administration followed a plan of containment, trying to keep IS from spreading. It failed spectacularly yesterday. This was the policy response many academics called for (see: What Isis Really Wants, The Atlantic), but it meant humanitarianism would be sacrificed for the gain of a slow death of the movement. IS shifted tactics yesterday by an outright foreign attack which was not considered a part of their methodology at the time of the Atlantic article. This shift should have been met with a different US response. Withdrawing from the conflict now, as you suggest, would indeed be different, but it would be interpreted poorly by our allies especially after claims from our president of solidarity. Leaving this problem solely in European hands could lead to even more violence in the mainland, terrorist or not, as well as lead to more destabilization in the region affected. It isn't a problem western interests can solve socially, and, if we agree with the French that al Assad is more trouble to the region than help because of his Shia faith, our diplomatic options are severely limited as well. This is, however, a problem western nations need to take on as it affects them via these terrorizing acts of war and by the refugees that are being forced towards Europe. Mobilizing troops is not a good solution unless it is treated and committed to like an actual war, but there will be many politicians who see it as the only solution. The US will certainly not help the situation by further helping destabilize the Assad regime, especially when Russian aid may prove essential to combating IS.