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 the population of the parent state. Historical reasons may also be responsible, i.e., that the territory previously formed part of the parent state. However, 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. However, 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.

Annexation, in international law, is the forcible acquisition and assertion of legal title over one state's territory by another state, usually following military occupation of the territory. In current international law, it is generally held to be an illegal act. Annexation is a unilateral act where territory is seized and held by one state, as distinct from the complete conquest of another country, and differs from cession, in which territory is given or sold through treaty.