The United States is facing possibly its worst intelligence leak since Edward Snowden’s 2013 flight to Moscow as a new batch of classified documents appeared on social media. More than 100 classified documents on Ukraine, China, the Middle East, and terrorism are now believed to be in the public domain after they were posted in an obscure internet forum last month. Last Thursday, White House officials said they were investigating the appearance of highly classified briefing documents on Twitter, and the Justice Department has launched an investigation into the leak. American officials said Russia or pro-Russian elements were likely behind...