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.”

In the wake of World War II, as American politicians grew increasingly fearful of the Soviet Union, Kennan warned that it was foolish to talk about foreign threats without defining national interests. If America didn’t first determine which chunks of the globe were worth defending, any place Moscow threatened would become important to the United States. The U.S. would effectively outsource its foreign-policy decision making. “Our opposition to Communist expansion is not an absolute factor,” argued Kennan in 1947. Rather, containment “must be taken in relation to American security and American objectives.”

Over the decades, Lippmann and Kennan saw their worst fears realized, as U.S. presidents increasingly equated containment with stopping Communism anywhere on Earth. In January 1950, Secretary of State Dean Acheson did not include South Korea in America’s “defense perimeter,” only to see his boss, President Harry Truman, rush troops there once Pyongyang attacked. Dwight Eisenhower didn’t consider South Vietnam important enough to defend militarily; Lyndon Johnson did. By the 1970s, some American hawks were warning about the consequences of Communist control of Somalia and Angola.

But the final blow came with the Cold War’s end. After the Soviet empire fell and America vanquished Iraq in the Gulf War, it became hard to imagine any foreign power seriously challenging America’s core interests. So hard, in fact, that the framework Kennan, Lippmann, and others had established no longer seemed a useful guide to American action. The first clue as to what would replace it came in a 1993 essay by National Security Adviser Anthony Lake, titled “From Containment to Enlargement.” The idea was that instead of defending the democratic world against Soviet advance, the United States would now push forward its boundaries. In eastern Europe, for example, the Clinton administration began admitting formerly Communist countries into NATO. “Enlargement” continued in a different form during the George W. Bush administration, which, in response to the 9/11 attacks, established pro-American governments in Afghanistan and Iraq, and dramatically expanded America’s military footprint in the Middle East and Central Asia.