My understanding of 'decline' in large parts of Africa was not down to warfare, but actually sudden environmental disasters which led to, or were accompanied by, diseases and crop pests.
The Gikuyu were often being raided by the Masai, but their decline came about after both their crops and their cattle were wiped out by various pests and diseases, and this was made worse by drought. From what I understand, an entire region (Kiambu) was effectively emptied, and anywhere between 25-50% of the indigenous population died off.
Unfortunately not too long after this cataclysmic event, Europeans showed up and thought the areas devoid of people hadn't been claimed (of course not like it would have mattered otherwise) and started to set up shop.
I've read of a few instances where tribes were discovered to have had an advanced political system, but they, similar to the Gikuyu, had suffered similar problems, with Europeans arriving to finish the job.
Interestingly on the subject of the Islamic conquests of large chunks of Africa, and the Arab Slave Trade, its estimated that just as many Africans were enslaved and shipped East, as had been enslaved by Europeans and shipped West. However its not difficult to see why the European Slave Trade generates far more interest.