Annexation

Annexation (Latin ad, to, and nexus, joining) is the administrative action[1] and concept in international law relating to the forcible acquisition of one state's territory by another state and is generally held to be an illegal act.[2] It is distinct from conquest, which refers to the acquisition of control over a territory involving a change of sovereignty,[3][4] and differs from cession, in which territory is given or sold through treaty, since annexation is a unilateral act where territory is seized and held by one state.[5] It usually follows military occupation of a territory.[1]

Basic Law: Jerusalem, Capital of Israel, passed by the Knesset on 30 July 1980
Two examples of unilateral annexation laws
The Flag of Hawaii over ʻIolani Palace is lowered following the Annexation of Hawaii by the United States (12 August 1898).
Civilians and coalition military forces wave Kuwaiti and Saudi Arabian flags as they celebrate the reversal of the annexation of Kuwait by Iraq (28 February 1991).

Annexation can be legitimized via general recognition by international bodies (i.e. other countries and intergovernmental organisations).[5][6][1]