Sam Heller lays out the difficulties and pitfalls of the impending illegal attack on Syria:

Not only would it be difficult to organise and message a multilateral intervention in a complex political and military space, but it is uncertain exactly how much damage a strike would have to inflict to achieve the desired goal and effectively deter the Syrian government from further chemical weapons use, while avoiding a dangerous Russian response or other form of escalation. In essence, a narrow, limited attack is unlikely to deter repeated chemical weapons use, while a broader one could have unintended and uncontrollable consequences. In particular, any attack risks hitting Russian and Iranian personnel commingled with their Syrian partners, triggering a broader confrontation. Even a strike that avoided doing so could be misread by Moscow or Tehran as designed to achieve more substantial goals.

The U.S. has become so used to taking military action without fear of meaningful retaliation that I worry that no one in Washington is taking the possibility of retaliation seriously enough. Last year’s attack on Syria was small enough that the U.S. was able to “get away” with doing it without sparking a larger conflict, but we shouldn’t assume that is how things will work this time. Relations with both Russia and Iran are extremely poor right now, and attacking the Syrian government is sure to make those relations much worse. It may not take much to provoke them into attacks on U.S. or U.S.-backed forces in Syria and elsewhere. That in turn would prompt demands from our own hard-liners to escalate again and again.

The U.S. has a poor track record of understanding how our adversaries see the world, so a brief thought experiment may be helpful. Imagine if the positions were reversed and Russia was the one threatening to attack an American ally that was in the midst of a civil war that included a Russian-backed insurgency. Wouldn’t the U.S. respond to an attack on an ally even if our own forces weren’t hit? We ought to assume that Russia will treat an attack on Syria the same way, and even if they don’t it isn’t worth taking the chance that they might. Punishing Assad is not worth the possibility of provoking war with Russia or Iran, much less both at the same time.