(derived from Latin ad, to, and nexus, joining) in international law is defined as the concept and unilateral administrative action of a state involving the forcible acquisition of another state’s territory. Annexation is generally considered to be a highly illegal act. It is distinct from conquest, which refers to the acquisition of control over a territory involving a change (surrender or relinquishment) of sovereignty. Annexation also differs from cession, a transactional process in which territory is given or sold through treaty. The unilateral act of annexation takes place when one state seizes and holds territory outside its sovereign jurisdiction, often following military occupation of the seized territory.

Annexation amounts to an act of aggression, forbidden by international law as a crime of state and subject also to individual responsibility within the jurisdiction of the International Criminal Court. International humanitarian law provides that, in the event that an Occupying Power annexes all or part of an occupied territory, protected persons therein shall not be deprived of the benefits of the Fourth Geneva Convention.

All states, whether Members of the United nations or not, are under legal obligation not recognize or cooperate with the illegal situation of annexation.