Outside of Syria, it also has another desired effect - underlining the differences between Mr Assad's opponents in the West. Last week, the hawkish US Republican senator, John McCain, who lost to Mr Obama in the 2008 presidential race, called on America to send in troops to secure factories such as al Safira. But Mr Obama shows no enthusiasm for doing so, and this weekend he even appeared to adjust his language slightly, saying that America would not permit the "systematic" use of chemical weapons. Critics pointed out that proscribing the use of chemical weapons on a "systematic" basis is not the same as proscribing their use altogether.