Let’s briefly review the American foreign-policy debates of the past year. Last August, President Obama declared that he would bomb Syria for defying his call to not use chemical weapons. Then, in a sharp about-face, he decided instead to work with Russia to dismantle the weapons, and was denounced as weak by hawkish critics. Obama’s supporters said he had done as well as he could have under the circumstances. Two months later, America and its allies struck an interim nuclear deal with Iran. Hawks called it appeasement. Obama’s supporters said it was as good as one could expect under the circumstances. Within hours of the deal, China claimed the right to monitor and possibly take military action against aircraft crossing a disputed area of the East China Sea. Hawks denounced Obama’s response as weak. The president’s supporters said it was as strong as possible under the circumstances. Then, in February, Russia began menacing Ukraine. Hawks called Obama’s response weak. His supporters said the president was doing all he reasonably could.

Each debate resembles the others, but occurs in splendid isolation. Today’s foreign-policy disputes rarely consider the way America’s response to one crisis might affect another. Adopt a tough stance on China’s air-defense zone, for instance, and Beijing is less likely to join the West in condemning Russia’s aggression in Ukraine. Severely punish Russia for that aggression, and Moscow is less likely to help America enforce sanctions against Iran. Take an ultra-hard line on Iran’s nuclear program, and Tehran is less likely to help broker an end to Syria’s civil war that the U.S. can live with. Instead of discussing each threat in isolation, America’s politicians and pundits should be debating which ones matter most. They should be prioritizing.

To understand how Americans stopped doing that, you need to go back to the middle of the 20th century. Over the course of the 1940s, as America lurched from isolationism to world war, foreign-policy strategists such as George Kennan and Walter Lippmann roughly outlined America’s core interests around the world. First, the United States must continue to prevent an enemy power from establishing a beachhead in the Americas, a principle set forth in the venerable Monroe Doctrine. Second, no enemy power should be allowed to dominate Europe, thus threatening what Lippmann called the “Atlantic highway” connecting the United States to Britain and France. Third, no enemy power should shut the United States out of East Asia; that’s what had precipitated war with Japan. Fourth, no adversary should block America’s access to Middle Eastern oil. But beyond these areas, U.S. interests were limited. “I am more and more convinced,” Lippmann wrote in 1943, “that it is just as important to define the limit beyond which we will not intervene as it is to convince our people that we cannot find security in an isolationist party.”...