Clinton says a no-fly zone, or area in which the U.S. would shoot down non-U.S. planes, would ensure humanitarian aid reaches civilians, pressure Russia and Iran to bring about a political resolution of the war in Syria, and speed the destruction of ISIS. But President Barack Obama has soundly rejected the idea, and critics warn it would cost billions of dollars, pull the U.S. further into the conflict, put American pilots at risk of being captured, tortured and killed, and create the potential for clashes with Russia that could lead to escalation or even war.