More than any other continent, Africa is wracked by separatists. There are rebels on the Atlantic and on the Red Sea. There are clearly defined liberation movements and rudderless, murderous groups known principally for their cruelty or greed. But these rebels share at least one thing: they direct their fire against weak states struggling to hold together disparate populations within boundaries drawn by 19th-century white colonialists....

Voters are expected to approve independence, and if it does, South Sudan will become a rare exception in Africa — a state that is reorganizing its colonial-era borders. It might even set a precedent for others.

In any case, it has already set off an agonizing debate, a half-century in coming, over the wisdom of trying to hold together the unwieldy colonial borders in the first place.

Even though many of those frontiers carelessly sliced through rivers, lakes, mountains and ethnic groups, few of the leaders who shepherded Africa to independence a half-century ago wanted to tinker, because redrawing the map could be endless and contested. So, on May 25, 1963, when the Organization of African Unity was formed in Addis Ababa, Ethiopia, it immediately recognized the colonial-era borders.

In hindsight, it is clear that the old boundaries often hurt prospects for state building. But back then, and even today for many Africans, the alternative of tiny ministates seemed even worse....