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This article applies principal component analysis to decompose transnational terrorism during 1970–2007 into common (worldwide) and idiosyncratic (country-specific) factors. Regardless of alternative thresholds and filtering procedures, a single common factor is related to individual countries' transnational terrorist events. Based on a conventional criterion, Lebanon's transnational terrorism is the key common driver of global transnational terrorist incidents. With a more conservative criterion, four additional countries—United States, Germany, Iraq, and the United Kingdom—are core countries in explaining cross-sectional correlation across 106 countries' transnational terrorism. The analysis shows that there is a marked cross-sectional dependence among transnational terrorist incidents worldwide. (JEL C38, H56)