If we knew how to reliably beat radicals we wouldn't be facing terrorism across the globe. Radicalism comes from the dramatic speed of globalization and the clashes that forces with the disenfranchised that are created in the stuggle.

Russia came in after the first chemical attack, negotiated the forfeiture of the chemical weapons (ostensibly), and stopped the US from drastically changing the course of the war. Assad was on the ropes back then, and our support of the rebels, groups who we had no real cohesive description of, could well have ended the war, created another vacuum of power, and given wide swaths of land to someone else.

ISIS has since become a larger threat than they were at the time because of the last power vacuum we created in Iraq. I have no desire to simply repeat the mistakes of the past by casually interfering in the affairs of foreign countries.

PS. Have a degree in middle eastern studies and did a lot of time over there fighting terrorism. Probably not as ill informes as you think.