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.

Military occupation, also called belligerent occupation or simply occupation, is the temporary control exerted by a ruling power's military apparatus over a sovereign territory that is outside of the legal boundaries of that ruling power's own sovereign territory. The controlled territory is then known as the occupied territory, with the ruling power being the occupant. Occupation is distinguished from annexation and colonialism on the basis that it is a power structure that the ruling power intends to keep in place only temporarily. In many cases, the occupant may establish a military government to facilitate the administration of the occupied territory, though this is not a necessary precondition for occupation to take place.