Part 12: Occupation by force is illegal
Since 1885, international law on territorial acquisition has taken profound changes. For unclaimed land (terres res nullius), effective, uninterrupted and continuous occupation with relatively large discontinuity can be compromised if it is compatible with the maintenance of rights and notification of such occupation to be sent to other powers through diplomatic channels. A name that had been established can be lost if the state having such title transfers it to another state, either by way of transfer or by acquisition by prescription.
In the first case, a treaty must be concluded to acknowledge the hand-over of power. In the second case, the mechanism of acquisition by prescription requires two conditions simultaneously. The state in name gives up its name and the territory will become abandoned (dereclictio).
The state concurrently exercises the ownership over the claimed territory, which could be considered as valid and enforceable against third countries in accordance with the required condition. This is a “consolidation by the impact of time” – a term used by Charles de Visscher – when exercising the sovereignty in an effective, peaceful and continuous manner. The acquisition by prescription, in this case, is none other than what C. Rousseau has pointed out – a label to cover up a devalued possession because of its illegitimacy.
Acquisition of territory by prescription almost always constitutes a basis for an act of violation . Therefore, in order to justify the acquisition by prescription, the second occupant must demonstrate its governance in a peaceful and effective manner without any flaw in respect of territorial sovereignty in the face of serious neglect of the first occupant.
Classic international law did not prevent the use of force to conquer a territory. However, the mode of territorial acquisition would require a conclusion of a peace treaty. In the early 20th century, the first attempts to limit the use of force in international relations were noted with the Drago doctrine (1902-1907). Finally, a decisive step towards banning the use of force was made by the adoption of the Treaty of Paris on August 26th, 1928 (Briand-Kellogg Pact).
Since then, the traditional rule of use of force has been condemned. This idea was reaffirmed in Article 2, Paragraph 4 of the UN Charter:
“All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the purposes of the United Nations.”
This principle has been developed and consolidated by the Resolution 2625 (XXV) of the UN General Assembly on October 24th, 1970 (Declaration on Principles of International Law Concerning Friendly Relations and Cooperation among States in accordance with the Charter of the United Nations):
“The territory of a State shall not be the object of military occupation resulting from the use of force in contravention of the provisions of the UN Charter. The territory of a State shall not be the object of acquisition by another State resulting form the threat or use of force. No territorial acquisition resulting from the threat or use force shall be recognized as legal.”
The UN Charter and resolutions of the UN General Assembly also condemn any threat or use of force as a means to resolve international disputes, including territorial disputes. The UN member states shall settle “their international disputes by peaceful means in such a manner that international peace and security, and justice, are not endangered.” 
By Dr. Nguyễn Hồng Thao
 Ch. Rousseau, international law, Sirey, 1977, volume III, p. 183.
 Fauchille, international law textbook, Paris, 1925, Volume I, p. 785.
 UN Charter, Article 2, Paragraph 3.
Source: Thanh Nien Daily