Irredentism is one state's desire to annex the territory of another state. This desire can be motivated by ethnic reasons because the population of the territory is ethnically similar to or the same as the population of the parent state. Historical reasons may also be responsible, i.e., that the territory previously formed part of the parent state. Difficulties in applying the concept to concrete cases have given rise to academic debates about its precise definition. Disagreements concern whether either or both ethnic and historical reasons have to be present and whether non-state actors can also engage in irredentism. A further dispute is whether attempts to absorb a full neighboring state are also included. There are various types of irredentism. For typical forms of irredentism, the parent state already exists before the territorial conflict with a neighboring state arises. There are also forms of irredentism in which the parent state is newly created by uniting an ethnic group spread across several countries. Another distinction concerns whether the country to which the disputed territory currently belongs is a regular state, a former colony, or a collapsed state.
A central research topic concerning irredentism is the question of how it is to be explained or what causes it. Many explanations hold that ethnic homogeneity within a state makes irredentism more likely. Discrimination against the ethnic group in the neighboring territory is another contributing factor. A closely related explanation argues that national identities based primarily on ethnicity, culture, and history increase irredentist tendencies. Another approach is to explain irredentism as an attempt to increase power and wealth. In this regard, it is argued that irredentist claims are more likely if the neighboring territory is relatively rich. Many explanations also focus on the regime type and hold that democracies are less likely to engage in irredentism while anocracies are particularly open to it.
