The U.S. Embassy in Beijing on Thursday morning warned citizens of a Christmastime terrorism threat against Westerners in one of the city's most popular expat districts. Beijing authorities put swaths of the city under lockdown, stationing armed guards on street corners and in pedestrian plazas. "The U.S. Embassy has received information of possible threats against Westerners in the Sanlitun area of Beijing, on or around Christmas Day," the embassy said in an email to American citizens living in Beijing. "U.S. citizens are urged to exercise heightened vigilance. The U.S. Embassy has issued the same guidance to U.S. government personnel." The...