Acquisition of sovereigntyA number of methods of acquisition of sovereignty are or have been recognised by international law as lawful methods by which a state may acquire sovereignty over territory. International law adopts much of Roman property law in regards to acquisition of sovereignty due to the underlying European civil law at the time of early discovery voyages such as Christopher Columbus. The basis of acquisition of states ownership of vacant territory therefore continues to apply, (and was often applied historically to land already possessed by indigenous populations).
Accretion refers to the physical expansion of an existing territory through geological processes, such as alluvion (the deposit of sediment) or vulcanism.
CessionA state may acquire sovereignty over territory if that sovereignty is ceded (transferred) to it by another state. Cession is typically effected by treaty. Examples of cession include the cession of Hong Kong Island and Kowloon, purchases such as the Louisiana Purchase and the Alaska Purchase, and cessions involving multiple parties such as the Treaty on the Final Settlement with Respect to Germany.
ConquestDirect annexation, the acquisition of territory by way of force, was historically recognised as a lawful method for acquiring sovereignty over newly acquired territory before the mid-1700s. By the end of the Napoleonic period, however, invasion and annexation ceased to be recognized by international law and were no longer accepted as a means of territorial acquisition. The Convention respecting the Laws and Customs of War on Land (Hague IV, 1907) contained explicit provisions concerning the protection of civilians and their property in occupied territories. The United Nations Charter also has related provisions.
In the case of United States v. Huckabee (1872), the US Supreme Court, speaking through Mr. Justice Clifford, said: "Power to acquire territory either by conquest or treaty is vested by the Constitution in the United States. Conquered territory, however, is usually held as a mere military occupation until the fate of the nation from which it is conquered is determined .... " Such a legal rationale naturally applies to all sovereign governments.
Effective occupation is the control of free newly discovered territory exercised by a power with no sovereign title to the land, whether in defiance or absence of a proper sovereign. Several cases in international law have dealt with what "effective occupation" entails.
In the words of the Eritrea/Yemen Arbitration Award:Also in the case of Mexico and France over Clipperton Island:In the case of the Netherlands and the United States in the Island of Palmas case, the arbitrator ruled:
Prescription is related to occupation, and refers to the acquisition of sovereignty by way of the actual exercise of sovereignty, maintained for a reasonable period of time, that is effected without objection from other states.