a formal act whereby a state proclaims its sovereignty over territory hitherto outside its domain. Unlike cession, whereby territory is given or sold through treaty, annexation is a unilateral act made effective by actual possession and legitimized by general recognition.

Annexation is frequently preceded by conquest and military occupation of the conquered territory. Occasionally, as in the German annexation of Austria in 1938, a conquest may be accomplished by the threat of force without active hostilities. Military occupation does not constitute or necessarily lead to annexation. Thus, for instance, the Allied military occupation of Germany after the cessation of hostilities in World War II was not followed by annexation. When military occupation results in annexation, an official announcement is normal, to the effect that the sovereign authority of the annexing state has been established and will be maintained in the future. Israel made such a declaration when it annexed the Golan Heights in 1981. The subsequent recognition of annexation by other states may be explicit or implied. Annexation based on the illegal use of force is condemned in the Charter of the United Nations.

Conditions may exist which obviate the necessity for conquest prior to annexation. In 1910, for example, Japan converted its protectorate of Korea into an annexed colony by means of proclamation. Preceding its annexation of the Svalbard Islands in 1925, Norway eliminated its competitors by means of a treaty in which they agreed to Norwegian possession of the islands. Annexation of Hawaii by the United States in the late 19th century was a peaceful process, based upon the willing acceptance by the Hawaiian government of U.S. authority.

The formalities of annexation are not defined by international law; whether it be done by one authority or another within a state is a matter of constitutional law. The Italian annexation of Ethiopia in 1936 was accomplished by a decree issued by the king of Italy. Joint resolutions of Congress were the means by which the United States annexed Texas in 1845 and Hawaii in 1898. See also conquest.