Critics of the Obama Administration point to several military options for intervention. These included one-time retaliatory military strikes in response to particular human rights abuses (such as the use of chemical weapons), the use of a no fly zone, the creation of "safe zones" on Syrian territory, and outright military support of some faction or the other of the Syrian rebels.
For several reasons, I remain deeply skeptical that any of these options would have improved the situation in Syria.
First, the recent history of military intervention hardly offers much support for the notion that U.S. military intervention in the Middle East improves matters. We were initially successful in enforcing regime change in Libya, but the result has been a failed state where ISIS and other extremists thrive. We were initially successful in overthrowing Saddam Hussein, but to call the result in Iraq (including the rise of ISIS) anything but a disaster would be disingenuous. In each case, the complicated tribal and sectarian divisions made military intervention--in the long term at least--the wrong solution. Most of the military leaders I worked with in the Pentagon, by the way, recognized this, and were opposed to intervention in Syria.
Second, with the exception of a one-time missile strike, most of the options above would have required a risky and extensive military campaign that would have required air strikes on hundreds of targets in Syria--often in areas near civilians. Syrian has a robust and modern air defense system (thanks, Russia), and any military option that required that we have unhampered access to Syrian territory or air space would have first required that we destroy the air defense system. I am fully confident that our Air Force and Navy would be up to the job, but the level of violence required would have been shocking. (This was likely the reason, by the way, that Trump used missiles, and not aircraft, for his strike on Syria).
Third, lest we forget, at the time military intervention was under active consideration, there was a complete absence of support by the American people (and much of the international community for that matter). Most voices in Congress were opposed. The British Parliament actually voted against intervention. And polling showed little appetite by the American public for another war in the Middle East. Given that most of these options required some sustained action over many years to be effective, the lack of support by the American people was critical.
(And even apart from all of the above, the legal justification for military intervention under both international law and U.S. domestic law was dubious at best. A sustained military operation would have required congressional consent under the War Powers Act, and the U.N. Charter does not permit unilateral military action absent a clear self-defense rationale).
To be clear, I still find the situation in Syria very troubling, and all of us should consider whether this was a case where some military action was warranted. I, for one, remain, deeply skeptical,