The fall of Col. Muammar el-Qaddafi seemed to vindicate Hillary Clinton. Then militias refused to disarm, neighbors fanned a civil war, and the Islamic State found refuge. Islamists were moving aggressively to seize power, and members of the anti-Qaddafi coalition, notably Qatar, were financing them. Mrs. Clinton would be mostly a bystander as the country dissolved into chaos, leading to a civil war that would destabilize the region, fueling the refugee crisis in Europe and allowing the Islamic State to establish a Libyan haven that the United States is now desperately trying to contain. “There was one arsenal that we...