I wrote my doctoral dissertation long ago on the effects the Vietnam War was having on U.S. foreign policy. It was turning us inward, preoccupied with domestic affairs and deeply cautious about overseas military intervention. This paradigm was likely to last for some time but end in a new bout of interventionism, I suggested.

Scholars have long detected cyclical or alternating patterns in history, including the history of U.S. politics. I explored how our foreign policy of a given period becomes rigid and wrecks itself by application ad absurdum, as it was then doing in Vietnam. As the established paradigm departs from reality, bitter domestic criticism grows until the opposite paradigm replaces it. This lasts until it too grows rigid and wrecks itself.

For example, World War I did not make the world safe for democracy and quarrelsome European allies wouldn’t repay their war debts. Public opinion, the media and Congress slouched into isolationism, swearing to never get involved in overseas messes again.

Of course, we could not withdraw from the world and were drawn back into overseas conflicts. Japan’s 1931 conquest of Manchuria brought Secretary of State Stimson’s “non-recognition” of this aggression — words without deeds — that left Tokyo unimpressed. In 1937, Japan began the conquest of all of China. Embargo of California oil to Japan clinched their militarists’ argument that the U.S. must be knocked out of the Pacific: Pearl Harbor.

The 1940 German Blitzkrieg through France started splitting U.S. opinion. Some saw that we would eventually have to engage, but the majority stayed isolationist, producing the short-lived America First Committee of 1940–41, which folded with Pearl Harbor.

American elites then plunged into almost unlimited intervention. We had been foolish in not responding sooner to aggression; that will never happen again. Stalin and the Soviet Union were the new aggressors, and the Cold War led to a series of showdowns. By early 1968, the Vietnam bog made Congress and the media question what we were doing there.

The paradigm then snapped back the other way. Only small interventions (Grenada, Panama) were tolerated. Expelling Iraq from Kuwait in 1991 was the first act of a new round of intervention. At first, Americans were enthusiastic. “Neoconservatives” (actually, Cold War liberals) preached expansion of democracy by wars that would pay for themselves. After years of inconclusive fighting in Afghanistan and Iraq, Americans wanted out.

An ideal foreign policy would find a sweet spot between too much and too little intervention. But it’s awfully hard to define and stick to this middle ground. Involvement, egged on by demagogues and the media, tends to grow. Being “a little involved” is like being a little bit pregnant. U.S. shipments to Britain twice brought us into war with Germany.

In 1952, political scientist Frank L. Klingberg quantified historical shifts and found average “moods” of 27 years of extroversion followed by 21 year of introversion. Not mentioning Indochina, Klingberg anticipated the current extrovert mood would end in the late 1960s, precisely what happened. Nixon pulled us (too slowly) out of Vietnam and ended the draft.

Adding Klingberg’s 21 years of introversion to 1968 gets us to 1989, the year of our Panama invasion. Two years later, we pushed Iraq out of Kuwait, initiating some 27 years of extroversion that put us into public rejection of the Afghanistan and Iraq wars.

God does not play numbers games with U.S. foreign policy, but this-worldly factors may explain the alternation. Prominently, a younger generation, enraged at the folly of the previous, enshrines and practices its paradigm. In 1940, the U.S. ambassador to London, Joseph Kennedy, wrote off Britain as finished; his son saw Britain as belatedly aroused. JFK then went on to overapply intervention.

Many trends arise from an emotionally charged younger generation rejecting the wisdom of the older generation. Those who fought World War II rejected isolationism so thoroughly that they made virtually the whole world our national interest. (It wasn’t.) Other factors may contribute to the cycle, such as war exhaustion, domestic politics and economic downturns.

We seem to be entering another noninterventionist phase that tries to avoid overseas conflicts. Today’s “restrainers” reject new wars in favor of domestic reform and reconstruction. If the above pattern holds, we could minimize overseas threats for some years and set up another catastrophe, leading to renewed interventionism.

Many foresee this coming in Asia, where China aims to hold sway. Neither China nor the U.S. want war over Taiwan or North Korea, but neither will abandon their respective small allies. Many urge preparation for collision. The Iran-Israel conflict could spill over until it involves the U.S.

The underlying problem is that the U.S. can neither control the outside world nor fully withdraw from it. Attempts to do either are bound to end in frustration.