George F. Kennan celebrated his 100th birthday earlier this year. The dean of U.S. diplomats is best known for his strategy of containment, which he first articulated in the so-called long telegram that he sent from Moscow in 1946—and soon thereafter unveiled in his 1947 article, “The Sources of Soviet Conduct,” published under the pseudonym “X.” Several conferences honoring Kennan have praised his enormous contribution to U.S. Cold War strategy, yet the most fitting tribute would be to apply his seminal theories to our present era—to examine the sources of terrorist conduct.

Containing the Soviet Union and fighting terrorism are strikingly different undertakings. Kennan examined the behavior of a sovereign state with defined borders, an established populace, a recognized government, and an official ideology. Terrorism, by contrast, does not operate within clear boundaries or abide by diplomatic niceties. Containment cannot deal with such an elusive adversary. But neither is war a fully adequate concept for addressing terrorism as an ongoing challenge. Terror is the tactic, not the adversary itself. To deal with terrorism over the longer term, we must go beyond the symptoms of the problem to address its underlying causes—which is precisely where Kennan's strategic logic takes us.

In his X article, Kennan argued that Soviet power was the product of both ideology and circumstance. Russia's antipathy toward the West was born of historical insecurity. In that context, communism was less a goal than a means—a way for Moscow to maintain control at home and spread its influence abroad. “This means that truth is not a constant but is actually created, for all intents and purposes, by the Soviet leaders themselves,” Kennan wrote. “It is nothing absolute and immutable.” This observation led Kennan to...