1 IN U.S. FOREIGN POLICY, IT ISN ALWAYS EASY to suss out good ideas from bad. Some bad ideas masquerade as neutral fact, only to be exposed later on. Others worm their way into strategic doctrines, guiding a wide range of policies that long outlast the original thought. Good ideas, meanwhile, can have bad effects and bad ideas can be used for good. Given this tangle, picking the worst foreign-policy ideas of the last 50 years may seem like knitting socks with fish line. But is its not impossible.

First, we need to establish what we mean by ideas. In their 1993 book, Ideas and Foreign Policy, the political scientists Robert Keohane and Judith Goldstein defined ideas as “beliefs held by individuals.” Within that broad category, they wrote, are two types: first, descriptive and causal beliefs (thoughts about how the world works and why) and, second, principled beliefs (ideas about how the world ought to look). The how and why beliefs can be evaluated according to their logic and accuracy. The ought beliefs can be evaluated according to whether, when realized, the world is on the whole a better place for the most people.

It is easy to say that ideas are bad when they simultaneously rest on faulty causal beliefs and are wielded in service of dubious principles. For example, King Leopold II of Belgium terrorized the Congo based on the unprincipled belief that African land belonged to whichever European nation could occupy it—and on the false causal belief that late 19th-century political elites, fresh from eradicating the trans-Atlantic slave trade, would neither notice nor object to the horrific reality of