Our military options.
Firstly we shouldn't.
But if the poison gas outrage is such that our political masters nudge us into involvement there is a way.
People talk about a no fly zone. Enforced with CAPs and SEAD.
But there is another way. Simply take out their (already depleted) air assets on the ground.
We (with our allies) have the intelligence assets to know where all their helicopters and fighter jets are. Then at 3AM one morning we could use stand off weapons and drones to neutralise them all.
Then we are seen to do something and we create a more level playing field on the ground. With no lives risked.
In a similar fashion we could degrade what is left of their armour. And then maybe selected strikes against the government C4ISTAR.
Then let the various opposing forces fight it out on the ground.
What is very pertinent recently is that the rebels have been putting a lot of effort to fight the Kurds in the north instead of fighting the government. Ethnic cleansing. They want a Sunni caliphate and everything else must be removed. In conquered territory they are enforcing strict sharia, which is a big shock to citizens used to a secular regime.