There is a certain similarity between the decolonization of Africa in the 1950s, 1960s and 1970s and the breakup of the USSR in 1991. In both cases, new sovereign states were formed on the precise territories of existing political divisions — the colonies in one case and the Soviet Socialist Republics in the other — with no adjustment of borders to account for ethnic imbalances.
The Western European powers that colonized Africa, of course, did not really care about “tribal” identities, except when it was convenient for them, as when the Belgians preferred to have the native administrators in Rwanda and Burundi (I’m using the present-day forms of the names, for simplicity’s sake) be Tutsi rather than Hutu. Traditionally Somali districts that had been previously incorporated into Ethiopia or Kenya, for political or administrative reasons, remained that way. In what is now Nigeria, existing political entities with vastly different cultures and histories were merged, at first (in 1900), into the two colonies of Northern and Southern Nigeria, and then (in 1914), into one Nigeria. Many of the bloody civil wars and other conflicts that have raged in Africa ever since can be attributed to this colonial disregard of ethnic identities.
In the Soviet Union it wasn’t quite like that, at first. Each of the SSRs was formed as the homeland of its leading ethnic nation, with provisions for minority nationalities, so that, in principle, the borders between them were reasonably representative of ethnic divisions. But the central role played by the Russian nation in the government of the Union made it desirable for the Kremlin to introduce Russians into the non-Russian republics, both as members the ruling elites (nomenklatura) and as settlers in large numbers (continuing a trend begun under the tsars). The current situation in Ukraine, where a large swath of territory inhabited mainly by ethnic Russians (and not merely Russian-speakers, many of whom are ethnic Ukrainians) was absorbed into the sovereign state replacing the Ukrainian SSR, is the direct outgrowth of this stubborn clinging to already-existing borders.