Turkey has “activated” a Russian-made S-400 anti-aircraft defense system to track F-16 fighter jets flown by several other NATO members, U.S. lawmakers and analysts suspect. “The whole problem with having the S-400 integrated is in order to do that, you have to betray some of the codes and some of the, basically, NATO technology to the Russians,” the American Enterprise Institute’s Michael Rubin said. “And it appears that that's what they've done.” Western officials have eyed Turkey’s turn to Russian arms dealers with alarm in recent years, but Turkish President Recep Tayyip Erdogan defied the threat of U.S. sanctions to...