"war between two African nations in the fourteenth century"
It may be that Kundera did not have any particular war in mind here, since his precise point is that such an event would have been beneath the notice of anyone in the “developed, educated” Western world. He might have picked the 14th century because it was only in the following one that Europeans, specifically Portuguese sailors sent out by Prince Henry “the Navigator,” began to explore the West African coast.
On the other hand, there were many lengthy wars over the course of the thousand-year life of the Kanem Empire in central Africa, between the 9th and 19th centuries. In the 1370s and 1380s, the Bulalas invaded from the east and south, and pushed the Kanuri people west, where they later formed a new empire in Bornu.
This illustration of Kanem-Bu warriors is from the 1892 book The earth and its inhabitants, Africa.