This study focuses on the problem of the effects of international threats on the cohesiveness of political systems. Specifically, it is a cross-national attempt to assess whether cohesive nations become more cohesive when externally threatened, and whether low- cohesive nations become less cohesive. These postulates, derived from theories of social psychology, are the primary hypotheses tested in this study. In order to test these hypotheses, the concepts of cohesion and external threat are translated into observational terms. Cohesion is indexed in terms of the level of conflict within nations; the greater the amount of conflict, the lower the cohesion. The level of internal conflict is conceived as a continuum. Eighty-three nations of the world are placed on this continuum according to the amount of internal aggression they experienced during the 1955-1961 period. A seven-point scale is developed for this purpose, with each point on the scale denoting a decreasing degree of cohesion. External threat is conceived as a function of international aggression. A country is considered threatened when it is the target of an aggressive act perpetrated by a foreign nation, or group of nations. Acts of external aggression are conceptualized similarly to acts of internal aggression. A seven-point scale ranging from low to extreme levels of external aggression is utilized. To obtain a measurement of the degree to which each of the eighty-three nations included in this study are threatened, all the external aggressive acts directed toward each nation during the 1955-1961 period are combined into yearly scores. An external threat score for the entire 1955-1961 period is also compiled. Additional hypotheses, designed to explore further the dynamics of the external threat-cohesion relationship, are also developed and investigated. These involve use of the variables of degree of permissiveness-coerciveness and level of development of political systems. Also, five distinguishable socio-cultural groupings--West European, Anglo-Saxon, East European, Asian, and Latin American--are separately examined to see if the relationship between external threat and cohesion varies for countries of differing sociocultural characteristics. A weak relationship is found between inter- national threat and the cohesiveness of political systems. This relationship, however, is predominantly in the direction of decreased cohesion in the face of external threats. Contrary to hypothesis, the high cohesive nations were found to exhibit a more pronounced tendency to become less cohesive when externally threatened than the low-cohesive nations, where no clear pattern was found. Accordingly, those groupings of countries examined which tend to be highly cohesive (permissive, modern, West European, Anglo-Saxon, and East European) showed a more marked tendency to become less cohesive in the face of international threats than those groupings low in cohesion (moderately coercive, transitional, traditional, Asian, and Latin American). Since correlation coefficients are predominantly low, no definitive conclusions are drawn in this study. Some interesting speculations, however, are offered Foremost, is the suggestion that the theories of social psychology, which predict that groups will become more cohesive when externally threatened, do not apply when political systems are used as the basic unit of analysis.