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.