Governments can legally acquire territory in a handful of recognized ways under international law: by treaty (cession), by natural land formation (accretion), by long peaceful control (prescription), by a genuine vote of the population living there (self-determination), and, historically, by claiming genuinely unclaimed land (occupation). Since 1945, taking territory by force has been prohibited outright, and other states have a duty to refuse to recognize any such acquisition. That is the short answer to how governments legally acquire territory; the rest is in the details, because a transfer that skips any of the requirements can be declared void.
Cession by Treaty
Cession is the most common lawful method. One state formally transfers sovereignty to another through a binding treaty, both sides consent, and the treaty defines exactly what land changes hands. The transferring state has to actually hold legitimate sovereignty over the territory in the first place.
Cessions have taken different shapes. France sold the 828,000-square-mile Louisiana territory to the United States in 1803 for $15 million.1United States Census Bureau. April 2023 – The 1803 Louisiana Purchase Russia sold Alaska in 1867 for $7.2 million.2Office of the Historian. Purchase of Alaska, 1867 Territory can also be swapped to resolve a border dispute or transferred as part of a peace settlement.
Consent has to be real. The Vienna Convention on the Law of Treaties provides that a treaty procured by coercing a state’s representative has no legal effect, and a treaty obtained through the threat or use of force against the state itself is void.3United Nations International Law Commission. Vienna Convention on the Law of Treaties That draws a hard line between voluntary agreements and treaties imposed at gunpoint.
Who Takes On the Debt
The predecessor state’s public debt tied to the ceded territory does not simply disappear. Under the 1983 Vienna Convention on Succession of States in Respect of State Property, Archives and Debts, the default rule for a standard cession is that an “equitable proportion” of the predecessor state’s debt passes to the successor, based on the property, rights, and interests that come with the land.4United Nations Treaty Collection. Vienna Convention on Succession of States in Respect of State Property, Archives and Debts The preferred approach is for the two states to negotiate a specific allocation. Newly independent states get a more favorable rule: no predecessor debt passes to them unless they agree to take it on.
Occupation of Unclaimed Land
Historically, a state could gain sovereignty by being the first to occupy and administer land that belonged to no one. International law called such land terra nullius, and the legal act of claiming it was “occupation.” This has nothing to do with the military occupation of another state’s territory. It meant planting a flag on genuinely unclaimed land and exercising continuous authority over it.
The doctrine was heavily abused during the colonial era. European powers declared inhabited territories to be terra nullius because the local populations lacked the kind of centralized government Europeans recognized. The International Court of Justice pushed back in its 1975 Western Sahara advisory opinion, finding that Western Sahara was not terra nullius at the time of Spanish colonization because the people living there had social and political ties to the land.5International Court of Justice. Western Sahara – Advisory Opinion
In practice, occupation of terra nullius is a dead letter today. Nearly every piece of land on Earth already falls inside some state’s recognized borders or is governed by international treaty, as Antarctica is. The principle still exists in theory. There is nothing left for it to apply to.
Accretion and Natural Land Change
Land shifts. Rivers deposit sediment, volcanoes create new islands, and coastlines move over decades. When new land forms gradually along a state’s existing territory, sovereignty extends to cover it automatically. This is accretion.
The word “gradually” carries the weight. When a river marks the boundary between two states, slow accretion on one bank shifts the boundary along with the river’s changing course. If the river suddenly jumps to a completely new channel, that is avulsion, and the boundary stays where it was. The distinction protects states from losing territory to a flood or earthquake.
New volcanic islands work similarly. When an underwater volcano produces an island inside a state’s territorial waters, the state gains sovereignty over it. Under the UN Convention on the Law of the Sea, a naturally formed island that can sustain human habitation or economic life generates its own territorial sea, exclusive economic zone, and continental shelf, like any other land.6United Nations. United Nations Convention on the Law of the Sea – Regime of Islands A rock that cannot sustain habitation gets a territorial sea but no exclusive economic zone.
Prescription
Prescription is the international-law cousin of adverse possession. A state acquires sovereignty by exercising peaceful, continuous, and public authority over territory for a long period, without meaningful protest from other states with competing claims. The theory is that if everyone accepts the situation for long enough, legal title should match the facts on the ground.
It is the hardest method to use. There is no fixed time limit, competing claims tend to surface early, and modern states are quick to protest encroachments on well-documented borders. Most disputes that might once have been resolved by prescription now go to treaty negotiation or to the International Court of Justice.
Self-Determination and Referendums
Since the mid-twentieth century, international law has recognized that the people living in a territory have a say in who governs them. The principle of self-determination, enshrined in the UN Charter, holds that all peoples have the right to freely determine their political status.7Legal Information Institute (LII). Self Determination (International Law) In practice, a population can vote through a referendum or plebiscite to join another state, become independent, or keep the status quo.
East Timor voted for independence from Indonesia in 1999 following a UN-supervised referendum, and the international community treated the outcome as legitimate.8UK Parliament. Peace Deals, Territorial Changes, and International Law Legitimacy is the sticking point. A referendum held under military occupation, without international monitoring, or with the voting population artificially altered does not carry legal weight. The International Covenant on Civil and Political Rights sets baseline standards: the vote must be genuine, conducted by universal suffrage, and held by secret ballot. When respected observer groups are absent, the international community is much more likely to reject the result.
Maritime Zones and Why Small Islands Matter
Territory is not just dry land. Under the UN Convention on the Law of the Sea, a coastal state exercises sovereignty over a belt of sea extending up to 12 nautical miles from its coastline, known as the territorial sea, where its authority is nearly as complete as on land, subject to a right of innocent passage for foreign ships.9United Nations. United Nations Convention on the Law of the Sea – Part II – Territorial Sea and Contiguous Zone
Beyond that, a contiguous zone extends up to 24 nautical miles from the baseline, where the state can enforce customs, immigration, and sanitary rules.9United Nations. United Nations Convention on the Law of the Sea – Part II – Territorial Sea and Contiguous Zone The exclusive economic zone reaches up to 200 nautical miles, granting sovereign rights over natural resources such as fish and undersea minerals without full sovereignty over the water itself.10eCFR. 33 CFR 2.30 – Exclusive Economic Zone
These zones are why acquiring even a small island can generate hundreds of thousands of square miles of exclusive resource rights, and why otherwise insignificant rocks and reefs become the subject of intense disputes.
Why Conquest Is Off the Table
For most of recorded history, conquest worked. A state invaded, occupied, and annexed its neighbor’s territory, and the international community eventually accepted the result. That era ended with the UN Charter, which requires all member states to “refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State.”11United Nations. Repertory of Practice – Article 2, Paragraph 4
The UN General Assembly reinforced the point in Resolution 2625, declaring that “the territory of a State shall not be the object of acquisition by another State resulting from the threat or use of force” and that “no territorial acquisition resulting from the threat or use of force shall be recognized as legal.”12United Nations General Assembly. Resolution 2625 (XXV) – Declaration on Principles of International Law Concerning Friendly Relations This is treated as a peremptory norm, meaning no treaty or agreement can override it.
Military occupation and annexation are separate things. An occupying power may control territory temporarily and bear administrative responsibilities under international humanitarian law, but it does not gain sovereignty.13International Committee of the Red Cross. Occupation It cannot extend its own laws over the territory or act as the legitimate government. Occupation is a regulated factual condition; annexation by force is flatly illegal.
The Duty of Non-Recognition
When a state tries to annex territory by force, other states have a legal duty to refuse to recognize the acquisition. The International Law Commission’s Draft Articles on State Responsibility codify this as a general rule: no state shall recognize as lawful a situation created by a serious breach of a peremptory norm. An aggressor cannot gain legal title to the fruits of its aggression, no matter how long it maintains control.
What Happens to the People Living There
A territorial change moves more than lines on a map. It changes the legal status of everyone living on the affected land. Practice has varied, but the general pattern is that the population receives some form of citizenship or residency, either through the treaty of cession itself or through later legislation.
The United States offers a clear record. The 1803 Louisiana treaty promised that inhabitants would be “incorporated in the Union” and admitted to “the enjoyment of all the rights, advantages, and immunities of citizens of the United States.” Similar language appeared in the 1819 acquisition of Florida from Spain and in the 1848 Treaty of Guadalupe Hidalgo with Mexico. In other cases, Congress granted citizenship by statute: the Hawaiian Organic Act of 1900 conferred citizenship on Hawaii’s residents, and the Jones Act of 1917 did the same for Puerto Ricans.14Library of Congress. Collective Naturalization (1800-1900)
Recognition and Domestic Incorporation
A legal basis for the transfer is only half the job. The acquiring state also needs the rest of the world to treat the new arrangement as legitimate. Recognition is a political act: other countries formally acknowledge the change through diplomatic statements, new diplomatic relations, or admission of the state (with its new borders) into international organizations. None of that is technically required for sovereignty to exist in theory. In practice, a territorial change most of the world refuses to recognize is precarious, and the acquiring state will struggle to enter treaties covering the territory or exercise jurisdiction without constant challenge.
Domestically, the acquiring state has to fold the new territory into its own legal system. That typically means the national legislature extending the country’s existing legal framework to the new land. In the United States, this has meant Congress passing an organic act that sets up a territorial government, defines its structure, and specifies which federal laws apply. The Organic Act of 1900 for Hawaii created a territorial government with an appointed governor, a judiciary, and an elected legislature, and granted residents citizenship and a nonvoting delegate to Congress.