It took ISIS and related allies' resorting to the ghastly medieval practice of beheading for the West and the outer world to get roused up. American public opinion, largely anti-war, turned around in an instant. Barack Obama, whose recent lackluster public appearances had given the impression, rightly or wrongly, that he really didn't enjoy being president, stepped before the United Nations last week as the master of the hour, announcing that he has formed a coalition to take the offensive against Isis.
ISIS has obliged in two respects. First, in carrying out beheadings, it drew forth the fury of the West. Second, in eliminating the Syria-Iraq border in favor of a larger Islamic Caliphate, it made moot whether attacks against it took place in Iraq or Syria. There is now a license for the U.S. and its Jordanian and Gulf allies to pursue their air attacks against Isis and its main concentrations, which are in Syria.
Thus the legitimacy for air intervention in Syria, which was lacking in 2013 when President Obama backed away from it, is now no longer an issue. Whether or not the campaign against Isis is successful, it was certainly necessary to have been undertaken.