Evidently, the Sunnis think that they can win.
Blowing up that key mosque was not exactly the most intelligent move that they could have made. At 4 to 1 odds, with the 4 being backed by USA firepower, it is a bad choice. If it goes all the way to full-fledged civil war, the Sunnis are likely to be exterminated. I had hoped that they would be smart enough to see the futility of starting a civil war, but they don't seem to be that smart. They will be sorry after it is too late. Perhaps full-scale civil war may yet be headed off. Time will tell.