When Attorney General William Barr stated "spying did occur" against the 2016 Trump campaign, most attention was focused on the FBI's surveillance of former junior foreign policy aide Carter Page. But the spying Barr was thinking of, and which he said may or may not have been legally authorized, is more likely to be that carried out by Stefan Halper, a former Republican operative and White House aide who became a foreign policy academic with close ties to both American and British intelligence. One could be forgiven for believing Halper was a creation of the spy novelist John Le Carré....