Barrack Obama started it. He famously warned Assad of crossing a “red line”. Then Assad crossed the red line. More recently, the Russians claim that the US crossed another red line when it bombed an Assad airbase.
Hmmm … lots of red lines and no end in sight to the civil war.
So what will happen next? The stumbling block to achieving a cease fire and then a protectorate over Syria has been what to do about Assad. The Russians insist that he stays. The west (before Trump) insisted that he has to go. It looked for a moment as if Trump was weakening on Assad, but then came the chemical attack. Scoring domestic political points trumped making consistent foreign policy. So off went the tomahawks.
My guess is that Trump will not go further and that this incident will be largely forgotten in a few months, just like the Obama missile strike was. Meanwhile, Assad will continue to try to strengthen his position in order to make kicking him out impossible, and the kurds and their buddies will try to kick ISIS out of Raqqa.
Neither of these things will end the conflict. So what will? That remains to be seen. Meanwhile, the red lines that should be remembered are indeed, trails of blood.