Western political and media elites reacted with horror to President Trump’s repeated statements that NATO is “obsolete” during the 2016 electoral campaign. They have also reacted with skepticism to more recent efforts by senior administration officials to affirm the U.S. commitment to NATO while pressing America’s allies to do more for their own defense. The critics forget both NATO’s history and — more fundamentally — confuse means with ends in U.S. national security. NATO is an instrument and, accordingly, something the United States can and should examine and seek to fix when it is not working properly. Mr. Trump has correctly understood that NATO isn’t doing its job. Post-Cold War history demonstrates NATO’s failure to adapt to changing circumstances and requirements. George W. Bush administration officials appropriately questioned the alliance’s contribution to U.S. operations in Afghanistan following the Sept. 11 attacks and NATO’s first-ever invocation of its mutual defense obligations under Article Five of the Washington Treaty. Later, NATO’s 2011 airstrikes against Libya illustrated considerable shortcomings as key allies proved unable to sustain the campaign for lack of precision bombs against a foe barely able to fight back.In 2014, when Russia annexed Crimea, NATO members all too readily opted to respond primarily through coordinated U.S.-European Union economic sanctions that predictably failed to deter subsequent Russian intervention in eastern Ukraine. Former President Obama bears no small responsibility for this, having declared in April 2014 that Russia could not be “deterred from further escalation by military force” at a time when decisive deployments of U.S. and NATO military forces in NATO member states surrounding Ukraine might well have affected Mr. Putin’s calculations. But Mr. Obama was far from alone among NATO leaders in his reluctance do this. NATO today has three major problems. First, the alliance has spent far more time discussing its membership than its purpose, leaving its goals unclear. If NATO is a defensive alliance, why did it intervene in Yugoslavia’s civil wars of the 1990s and launch airstrikes in Libya? Neither threatened NATO members with attack. If NATO seeks to stabilize Europe and Eurasia, how did NATO officials expect to do that without a security architecture that incorporated Russia on mutually acceptable terms? Conversely, if NATO sees Moscow as an existential danger and aims to contain and deter Russia, why do so few alliance members meet minimal standards for defense spending and military readiness?