NATO endures on American backing while many allies demand U.S. action abroad but withhold it when asked, exposing a widening gap between rhetoric and responsibility. NATO members are not legally required to join any member’s military operations that are not formally sanctioned by the alliance or not aimed at protecting the homelands of the membership. But they often do just that. Some NATO members joined the Americans in Afghanistan and Iraq on the theory that, in the post-9/11 environment, the Taliban and Saddam Hussein were dangers to all Western security. They followed the precedent set by America’s 1999 intervention in...