In my opinion US wont attack syria. Firstly, without backing of UN's Security Counsel. Secondly, domestic view of americans has been rising against poking nose in someone else's mess, therefore she can not afford to drag itself in another pointless adventure. Thirdly, If Assad is toppled, the list of candidates to fill the vaccum is not so pleasant. Fourthly, according the US intelligence reports, the evidence is not substantial enough to convict Assad. US has seen the consequences of luanching attack on the basis of questionable intelligence in Iraq so she would refrain from repeating the same mistake.
Against all odds, if US carries out the attack it will have far-reaching effects for the whole region. Putin has already ordered military action against Suadi Arabia, if west attacks Syria.
http://iranian.com/posts/view/post/19821
God forbid, if this happens so, it will no doubt be a WW III.