On a recent Monday, the Department of Justice announced the arrest of an 18-year-old man, Davin Daniel Meyer, on charges of attempting to provide material support to a foreign terrorist organization. Meyer had been arrested the previous Friday, July 14, at Denver International Airport as he tried to board a flight to Turkey. Meyer thought he was going to the Turkish city of Ankara for a rendezvous with members of the Islamic State terrorist group. In a press release announcing the arrest, the Justice Department said that Meyer had been caught after he had “pledged an oath of allegiance to...