Did the United States make a huge mistake by not aggressively supporting and arming the Free Syrian Army back in 2011-12? Did this decision produce a power vacuum that prompted the rise of ISIS in Iraq? Marc Lynch says no to the first question:
The academic literature is not encouraging. In general, external support for rebels almost always make wars longer, bloodier and harder to resolve....Worse, as the University of Maryland’s David Cunningham has shown, Syria had most of the characteristics of the type of civil war in which external support for rebels is least effective.
....Syria’s combination of a weak, fragmented collage of rebel organizations with a divided, competitive array of external sponsors was therefore the worst profile possible for effective external support....An effective strategy of arming the Syrian rebels would never have been easy, but to have any chance at all it would have required a unified approach by the rebels’ external backers, and a unified rebel organization to receive the aid. That would have meant staunching financial flows from its Gulf partners, or at least directing them in a coordinated fashion. Otherwise, U.S. aid to the FSA would be just another bucket of water in an ocean of cash and guns pouring into the conflict.
And he says almost certainly no to the second question as well:
The idea that more U.S. support for the FSA would have prevented the emergence of the Islamic State isn’t even remotely plausible. The open battlefield and nature of the struggle ensured that jihadists would find Syria’s war appealing. The Islamic State recovered steam inside of Iraq as part of a broad Sunni insurgency driven by Prime Minister Nouri al-Maliki’s bloody, ham-fisted crackdowns in Hawija and Fallujah, and more broadly because of the disaffection of key Sunni actors over Maliki’s sectarian authoritarianism. It is difficult to see how this would have been affected in the slightest by a U.S.-backed FSA (or, for that matter, by a residual U.S. military presence in Iraq, but that’s another debate for another day). There is certainly no reason to believe that the Islamic State and other extremist groups would have stayed away from such an ideal zone for jihad simply because Western-backed groups had additional guns and money.
Had the plan to arm Syria’s rebels been adopted back in 2012, the most likely scenario is that the war would still be raging and look much as it does today, except that the United States would be far more intimately and deeply involved.
Supporters of more aggressive military action have an easy job: all they have to do is point out what a mess the Middle East is today. And they're right: it's a mess. The obvious—and all too human—conclusion to draw is that things would be better if only we'd done something different three years ago. And the obvious different thing is more military support for the Syrian rebels.
But this is a cognitive error. Most likely, if we had done something different three years ago, the entire region would still be a mess—possibly a much worse mess—and we'd be right in the middle of it, kicking ourselves for getting involved in yet another quagmire and wondering if things would have gone better if only we'd done something different three years ago. Except this time the "something different" would be going back in time and staying out of things.
It's human nature to believe that intervention is always better than doing nothing. Liberals tend to believe this in domestic affairs and conservatives tend to believe it in foreign affairs. But it's not always so. The Middle East suffers from fundamental, longstanding fractures that the United States simply can't affect other than at the margins. Think about it this way: What are the odds that shipping arms and supplies to a poorly defined, poorly coordinated, and poorly understood rebel alliance in Syria would make a significant difference in the long-term outcome there when two decade-long wars in Afghanistan and Iraq barely changed anything? Slim and none.
Read Lynch's entire piece for more detail on why intervention would almost certainly have been doomed in Syria. And, once again, I recommend the five-minute primer above from Fareed Zakaria about what's at the core of the Syrian civil war and why it's highly unlikely that we should be involved. It's well worth your time.