In a corner of northwestern Syria packed with nearly 3 million people, the government and its opponents are preparing for a final, bloody showdown. The campaign for Idlib, the opposition's only remaining stronghold in the country and now a refuge for over a million displaced Syrians, is likely to be the last major theater of battle after seven years of brutal civil war. It is also potentially the most dangerous. The U.N. and aid workers are bracing for disaster, warning that up to 800,000 people are in danger of renewed displacement if a government offensive gets underway. A massive military...