In Syria’s last opposition stronghold, the worst fears of millions of people are coming true. With the help of brutal Russian airstrikes, Syrian government forces have seized about a third of Idlib Province over the last two months, pushing over 900,000 of the region’s 3.5 million people out of their homes and north toward the nearby Turkish border, where another 800,000 displaced people already live in crude, overcrowded camps. The current wave of refugees fleeing Idlib, about 80 percent of whom are women and children, is now the largest exodus of Syria’s nine-year conflict. Aid agencies are overwhelmed, and food...