The collapse of Czechoslovakia, the Soviet Union, and Yugoslavia resulted in the disintegration of the three multiethnic, federal states of Eastern Europe into their constituent republics. Each of these, with the sole exception of Bosnia-Herzegovina, was constituted along national lines and contained a titular nation. Almost none of the republics corresponded to the ethnically defined nations: either a significant number of individuals lived outside of 'their' republic or the republics contained significant minority groups. While the federal states existed and communist authority was intact, this fact mattered little. However, the transformation of these administrative boundaries into international borders meant that many individuals found themselves separated from their national homeland. The often arbitrary and artificial nature of the republics' borders, combined with the reality that they often do not match the ethnic boundaries of their respective regions, raises the specter of ethnic conflict and attempts by ethnic groups to secede from these newly formed states. In certain cases, one successor state may forcibly intervene in another state on behalf of its co-nationals for the purposes of annexation. This is the phenomenon of irredentism.

The term irredentism is derived from the Italian word irredenta (unredeemed) and originally referred to the political movement during the mid-1800s to early 1900s to detach Italian speakers from Swiss and Austro-Hungarian control and bring them into the newly formed Italian state. Those Italian-speaking territories outside Italy were considered 'unredeemed'. Modern usage denotes territorial expansion based upon an ethnic, national, or historical rationale. Although scholars are often divided on a precise definition of this phenomenon, I define it as follows: attempts by existing states to annex territories of another state which their co-nationals inhabit. Irredentism is similar to secession but different enough to treat it as a separate concept. Secession can be defined as "an attempt by an ethnic group claiming a homeland to withdraw with its territory from the authority of a larger state of which it is a part." To paraphrase Donald Horowitz, irredentism refers to subtracting from one state and adding to another, while secession is subtraction alone. Thus, the two distinguishing features of irredentism are: a pre-existing state and territorial claims upon another state based upon ethnic affinity.

Clearly, however, not all potential irredentists pursue irredentist policies. In the post-Cold War period, the record is mixed: Serbia, Croatia, and Armenia all attempted to unite their nations through force; Russia and Hungary, by contrast, did not. In addition, there were numerous cases throughout the world in which states resisted the temptation to launch nationalist wars. The purpose of this study is to answer the following questions: Why do some states initiate irredentist conflicts while other do not? Why are some states successful while others fail? What could cause a state to relinquish its territorial claims and accept an international border which separates members of the same nation or ethnic group?

This study attempts to answer these questions by taking a two-level approach to the phenomenon of irredentism: domestic and international. It will be argued that potential irredentists confront two potentially conflicting forces: domestic nationalism and the policies of the international community. The first factor, the level of ethno-territorial nationalism, is defined as the degree to which the state advocates and pursues policies based upon the premise that "the political and national unit should be congruent." This form of nationalism prods a state to undertake an irredentist project; as it rises, the state will become more of an irredentist. The idea of reclaiming one's diaspora and creating a 'Greater X' is a powerful motivating force for national-based collective action and can be promoted by either elites (top-down), the masses (bottom-up), or a combination of both.

On the other hand, relevant external actors, referred to here as the 'international community', can play a countervailing role. Since the adoption of the United Nations Charter in 1945, a major premise of the postwar international order has been the inviolability of borders and a general prohibition against territorial aggrandizement: Article 2(4) of the UN Charter explicitly states that all states "shall refrain from the threat or use of force against the territorial integrity or political independence of any state." An irredentist project clearly violates this principle. But, because there is no independent, overarching enforcement mechanism, enforcement of this norm is left to the states of the international system. The degree to which relevant external actors are acquiescent to the policies of an irredentist state will significantly determine the success of such a policy. I call this countervailing variable the level of international toleration.

The convergence of these two variables determines this study's dependent variable: the level of irredentism - defined as the irredentist outcome or the relative success of an irredentist project. If the impact of the international community were nil, then this level of nationalism would translate almost directly into the level of irredentism (barring defeats on the battlefield and other military considerations). However, because relevant external actors play a significant role, the level of international toleration tempers or restricts the policies of an irredentist. Therefore, this variable focuses on the outcome of an irredentist conflict.

This study utilizes several case studies to establish a connection between international pressure and a change of policy by the irredentist state. Post-Cold War Serbia, Croatia, Hungary, and Armenia are all examined in depth, while three historical examples, India, Indonesia, and Somalia, are also investigated. The case of Russia is also given special treatment.

The findings of this project yield meaningful policy implications: the importance of the international community's policies on the initiation and outcome of irredentist campaigns suggests that international actors must take a more active stance to prevent such conflicts. Rather than considering international ethnic conflicts intractable problems which must be avoided, outside powers can and do make a difference.