Why did new countries keep the borders their colonisers drew?

The short answer is that the alternative looked worse. When a colony becomes independent, someone has to decide where the new country ends. International law has a default answer, with a Latin name: uti possidetis juris. It says that newly formed states inherit the internal borders that their preceding dependent area had before independence. The old administrative line becomes an international border.

The phrase began in Roman courts, where it settled quarrels between neighbours about who held a piece of land. The judge preferred whoever was actually occupying it, unless he had taken it by force, by stealth or as a favour. The winner had not proved he owned the land, only that he had the better right for the moment. In international law the same words later took several meanings. After Spain's American empire broke up, new states often told arbitrators to use the uti possidetis at the date of independence, although a definite historical line could rarely be found. Later the principle was extended to Africa, Yugoslavia and the Soviet Union.

Africa is the best-known case. At an Organisation of African Unity conference in 1964, most colonies decided to keep their colonial borders, because they feared civil wars and regional instability. New states may still change the lines by mutual agreement with their neighbours.

The rule has critics. Some argue it collides with the principle of self-determination. Castellino and Allen argue that several separatist conflicts, such as Biafra, East Timor, Katanga, Kosovo and Rwanda, can be traced to blind insistence on borders they call artificial.