HANGEDIGI, Turkey—From above, the new border wall separating Turkey from Iran looks like a white snake winding through the barren hills. So far it only covers a third of the 540-kilometer (335-mile) border, leaving plenty of gaps for migrants to slip across in the dead of night. Traffic on this key migration route from central Asia to Europe has remained relatively stable compared to previous years. But European countries, as well as Turkey, fear the sudden return of Taliban rule in Afghanistan could change that. Haunted by a 2015 migration crisis fueled by the Syrian war, European leaders desperately want to avoid another large-scale influx of refugees and migrants from Afghanistan. Except for those who helped Western forces in the country’s two-decade war, the message to Afghans considering fleeing to Europe is: If you must leave, go to neighboring countries, but don’t come here. “It must be our goal to …