The thesis explores political-security aspects of a late twentieth century phenomenon: the existence of many diminutive and weak, yet ostensibly sovereign and independent, states. The thesis addresses two main questions concerning these "microstates" (with a population of less than one million). What are the principal sources of microstate vulnerability? How best can we conceptualize microstates' security dependence on larger powers? Foreign military intervention in response to microstate security crises throws these dual issues into sharp relief. The study covers all 55 independent microstates during the years 1960 to 1989, from the conventional beginning of decolonization to the end of the Cold War. Particular attention is paid to four representative case studies: Vanuatu (Papua New Guinea's 1980 intervention to quell a secessionist rebellion); The Gambia (Senegal's 1981 suppression of a coup attempt); Grenada (Cuba's role in respect of the 1983 American invasion); and the Maldives (India's thwarting of a 1988 mercenary attack). The thesis draws on an original data base of microstate security crises, a wide range of academic literature covering International Relations theory and small states, and field work. It tests the propositions that certain typical political, geographical and economic characteristics of microstates played a key role in determining vulnerability to security threats, and that microstates' dependent relationships with larger powers are in keeping with the patron-client model of such unequal associations. The thesis concludes that a mix of typical microstate features heightened their vulnerability, notably disadvantageous colonial legacies; tendencies towards "exaggerated personalism", "leadership longevity", and unrestrained executive power; remote insularity; and extreme government resource constraints. The patron-client model was found to be a useful conceptualization of dependent security relationships with larger powers, in terms of the pervasiveness of the latter's engagement in the microstates, conformity in foreign policy and mutual benefit, but the criterion of informality was frequently not met.