a lot of African countries were invented by European colonizers who didn't know anything about locals.
StrategyPage has a discussion on the up coming plebiscite in Sudan, voting if the south (black, Christian/animist) should secede from the north, (Arab/Muslim)
...Colonial powers drew most of the borders in the 19th and 20th centuries. The borders-drawn-in-parlors often divided tribes and sometimes made very little on-the-ground geographic sense. If South Sudan votes for independence (secession the northerners call it) the thinking goes that this will cause a chain reaction, first in the Grand Sahel (Darfur being another possibility) then throughout the rest of the continent. ...
The article has little to do with Zimbabwe, but it also mentions the problems with the central African wars, where of course Zim troops were fighting.