President Trump is often accused of creating a needless rift with America’s European allies. The secretary-general of the North Atlantic Treaty Organization, Jens Stoltenberg, expressed a different view Thursday when he told a joint session of Congress: “Allies must spend more on defense—this has been the clear message from President Trump, and this message is having a real impact.”

Mr. Stoltenberg’s remarks reflect a growing recognition that strategic and economic realities demand a drastic change in the way the U.S. conducts...