Don't give weapons to rebel groups if you're not going to guarantee that the Civil War will end because of it. Enforce a no fly zone so that the Syrian air force doesn't terrorize civilians or gas children. Optional: be willing to negotiate with Assad regime so that, in exchange for democratic elections and release of political prisoners and handing over WMDs, UN coalition forces secure country against fundamentalist death cults. And lastly, when the region becomes a giant proxy war for various interest groups who do not have the restraint becoming of civilized nations, be willing to utilize military hegemony and enforce the Pax America by bringing together the various parties and establishing who-gets-what.
I'd start my critique of Obama with the tepid escalation strategy in Afghanistan. A strategy that Trump seems to have doubled down on. But that is not really relevant.
On Syria, so then you are stuck in the position of genocide or potential regime change with further escalatory potential, which are both not exactly desirable strategies.
On the no-fly zone, here is Susan Rice's comment on that. I think it is a pretty fair counter-argument to the sort of arguments people like Evelyn Farkas were going around talking about during that same time:
34:34 Charlie Rose: So you look for alternatives and couldn't find them, even though they are.
34:37 Susan Rice: We didn't find suitable alternatives, satisfactory. Of course, there were people advocating for a "no fly."
34:43Charlie Rose: Exactly.
34:44 Susan Rice: So let's talk about that, for example. What would a "no fly" zone have done? The "no fly" zone, the concept was to create a swath of territory, most of the time it was discussed on the northern border of Syria with Turkey, where people could flee the fighting and have relative security, okay. That was the concept. "No fly" zone, however, and, by the way, just to be clear, and try to prevent Assad from using air power, barrel bombs, whatever, against civilians, we could have done that, but it would have been at great cost to the counter ISIL campaign in terms of diversion of assets and resources. We have --
35:25 Charlie Rose: We don't have enough power to do both?
35:27 Susan Rice: We're doing a lot of things in the world simultaneously. And no, the answer is, had we chosen to enforce the significant "no fly" zone, we would have taken assets away from the counter ISIL fight in Iraq and Syria. That's the choice we could have made. It wasn't one we thought was directly serving our proximate interests. Moreover, you can't just have folks, you know, protecting people on the ground through air power in the sky. You need to have somebody on the ground providing that protection. And there wasn't, NATO country, not Turkey, not anybody at that time, willing to provide that kind of protection. So, it was an idea that sounded good in theory, but when you peeled it back and talked about what would it actually entail, what diminution of our support for the ISIL campaign. Who is going to provide the ground force? How many air caps would that require? It didn't end up making sense.
I think that is what I am talking about when I mention asterisk when it comes alternative strategies. The US did not have the coordinating resources to commit, or the commitment from allies, to effectively enforce a no-fly zone from the ground without a diversion or significant and notable escalation of resources. I don't have a problem with people saying that trade-off was perhaps the superior strategy, acknowledging the trade offs and risks that come with that strategy, but to frame it like it is an obvious blunder is a bit disingenuous IMO.
Which gets to you point about UN forces, its hard to anchor an alternative strategy around cooperation from people that were not providing any significant military cooperation at the time. And Assad had no interest in stepping aside, which is the major part of why this escalated in the first place. Why this continues to be an elusive and difficult thing to figure out IMO. We didn't start the Civil War, and the country was going to be on a path of destabilization with or without our intervention. You had the splitting of the free Syrian Army, that fracturing into ISIS factions, which we were primarily focusing resources on, the emergence of multiple militias and rebel groups. Without any intervention, there is a strong case to be made that Assad and their Russian allies see that as an opening to further bulldoze and engage with the use of illegal chemical weapons and draconian pacification tactics to bring the country under heel.