People wouldn't know what would have hit them. The tourists and the terrorists alike
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While I sound like a broken record on this issue, we should have intervened in some meaningful way at/near the start of this crisis. Syria is a massive nation that stands at the center of the Middle East, people thought (and probably still think) they can get away with doing nothing about Syria when the fact remains there is an emerging lost generation displaced by war that will probably cause numerous problems in the next 40-50 years. Thankfully (some) Europeans are taking in (some) refugees, but the humanitarian issue will evolve into further realpolitik problems for the next couple of decades.
Anyways since the start of the crisis has long since passed - there will be a larger mess from any action than if we had acted at the start. The Afghan model was incredibly successful due to having pre-existing local forces with vested interests in retaking traditional territory, we do have the capability to do more with the Kurds and Iraqis in regards to Syria's eastern front. On the southern/western front the troops we are training are struggling to accomplish too much by themselves.
No one wants to hear it, but the truth is the best solution might actually be a multi-nation military intervention. The result will still be a mess - extremists would definitely still exist after said intervention, moderates who survived and have since been hardened by the war may do things we don't approve of, secular retaliations against minorities that supported/support Assad might happen, etc. Point is intervention will/would probably have all of these non-desirable side effects, but it would cause the civil war to wrap up faster and allow Syria to proceed on the path to recovery faster. As it stands without intervention, this conflict could go on for another half decade and then recovery for Syria will take at least three decades. Intervention could cut down this time needed for recovery maybe in half while reducing numerous threats of terrorism and spillover violence to nations with interests at stake.