The Free World and the United States of America are outraged that the Syrian regime killed more than 1,600 of its people with chemical weapons. That will not stand, the leaders of the Free World declared. Only conventional weapons are legal and are allowed to kill your people. So Mr. Assad, if you don't want us to bomb your bedroom, you better stick to legal ways you had used before to kill over a 100,000 of your people.
Almost 90 years ago the world agreed that there are plenty of other ways to kill each other besides the disgusting and smelly chemicals. (Although we still do it with pesticides).
So here is the Red Line: conventional killing only! Capisce?