Prior to the rise of ISIS in 2014, the United States generally sought to contain the conflict, as efforts at international diplomacy failed to resolve it. The Obama administration advocated for Assad to step aside, but was reluctant to send weapons or funds to the rebels opposing him, out of fear that they would fall into the hands of Islamists and radical jihadists among them. In 2012, Obama also famously set a “red line” regarding chemical weapons, saying that their use would change his calculus on U.S. strategy there. But when Assad used sarin gas on civilians in 2013, Obama opted, instead of using force, for an agreement with Russia to destroy Assad’s stockpiles of chemical weapons. The U.S. started bombing Syria for the first time a year later, hitting not regime targets but targets associated with ISIS. It has continued bombing ever since.President Trump recently said the U.S. would leave Syria “very soon”—even as his military advisers were planning to send additional troops. By the time he spoke, the U.S. presence in the country had grown to some 2,000 troops; news reports say Trump wants them out within six months.Then came the suspected chemical attack of last weekend, to which Trump retaliated with strikes against the regime, as he did to a similar attack last year. This means the United States has once again expanded its mission beyond counterterrorism.