Tariffs are on, until they are not. Military force is an option … and then it’s off the table.

Erratic behavior and unpredictability is having a moment in foreign policy circles. In the White House and elsewhere, it is seemingly being viewed as a strategic asset rather than a weakness.

But it is far from a new strategy. Wild threats, sudden policy reversals and intentionally confusing language have long been used to keep adversaries off balance and gain leverage.

In fact, the concept has its own name in international relations: “madman theory.” As outlined by Cold War strategists Daniel Ellsberg and Thomas Schelling, it holds that projecting a readiness to take extreme action can shape an opponent’s calculations by heightening fears of escalation.

While the theory was meant to be explanatory, in the sense that observers used it to explain apparently irrational behavior, it has sometimes been used in a prescriptive way, as an approach consciously adopted by leaders.

The madman theory has historical roots going back to Machiavelli, but it is most closely associated with Richard Nixon, who, as incoming president, reportedly used the term to explain his approach to trying to force North Vietnam’s surrender in the Vietnam War.

Historians see evidence of the theory’s limited applicability in episodes such as Nixon’s 1969 placing of the U.S.........

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