There are several main ways of acquiring territory under international law

There are five ways of acquiring territory under traditional international law: pre-emption; prescription; annexation; conquest; and cession.1. Pre-emption.

Pre-occupation is the act of a state consciously acquiring sovereignty over land that is not under the sovereignty of any other state. Pre-occupation must have two conditions: (1) the object of pre-occupation must be unclaimed land, that is, land that does not belong to any state, or by the original state explicitly abandoned. (2) Pre-occupation should be "effective occupation": firstly, the State should have the intention of acquiring sovereignty over the terra nullius, which should be manifested openly; secondly, the State should have actual control over the terra nullius, including the adoption of legislative, judicial and administrative measures, the establishment of institutions, and the marking of sovereignty, and other appropriate actions.

After the eighteenth century, the mere act of discovery of terra nullius was generally regarded as constituting only a prima facie right to temporarily prevent the occupation of another State, while the actual or effective occupation referred to above could constitute a pre-emption and acquisition of sovereignty over the land. The mode and degree of effective occupation required should be commensurate with the level of human activity at each stage of history. Statically, there is no object of pre-occupation in the world nowadays, and thus the principle of pre-occupation is most useful today as a means of clarifying and solving certain historical problems.

2. Statute of limitations.

Here is the statute of limitations on acquisition. This concept in traditional international law, borrowed from civil law, refers to the acquisition of sovereignty over the territory of another State as a result of the State's open, undisturbed, and long-continued possession of that territory. However, because the statute of limitations here does not ask whether the occupation itself is unlawful or not, and because of the two problems concerning the failure to determine the duration of the statute of limitations on acquisition, the application of the statute of limitations has traditionally been highly controversial. It is now essentially of no general application.

3. Addition.

Addition refers to the increase in the territory of the state due to the emergence of new naturally occurring or man-made land, which includes both natural additions such as estuarine deltas and upwelling beaches, and artificial additions such as sea enclosures. Artificial additions cannot be detrimental to the interests of other States. Addition has traditionally been recognized as a legitimate form of territorial acquisition in international law.

4. Conquest.

Conquest, also known as annexation, is the taking of all or part of the territory of another State by direct force of arms and its incorporation into one's own territory, thereby acquiring sovereignty over that land. Conquest does not require the conclusion of a treaty, but a valid conquest must fulfill two conditions: (1) the conquering state's formal expression of annexation of the defeated state's territory; (2) the defeated state's abandonment of its attempts to regain the lost territory, or the defeated state and its allies' expression of submission and abandonment of all resistance. Conquest is based on the legality of war, and it is clearly abandoned by modern international law.?

5. Cession.

Cession is the transfer of part of the territory of one state to another under a treaty. Cessions are divided into compulsory and non-compulsory cessions. Forced cession is a country by force to sign a treaty to force other countries to cede territory, usually as a result of war or war coercion. Non-compulsory cessions are those in which a State voluntarily transfers part of its territory to another State by means of a treaty, including sale, gift and exchange. Compulsory cessions have lost their legitimacy with the abolition of war in modern international law. Non-compulsory cessions remain legal and valid.