Even before his presidency began, Barack Obama articulated a foreign-policy course markedly different from that of his immediate predecessors. Not only did he present himself as the anti-Bush, but he also indicated that his administration would take a different approach to national security than had the Clinton administration. He was to be, in his aides’ terms, a “realist,” much in the mold of George H. W. Bush. As his then–chief of staff Rahm Emanuel put it in 2010: “Everybody always breaks it down between idealist and realist. If you had to put him in a category, he’s probably more realpolitik, …