The Mossi states have existed for at least 500 years; the exact dates and origins of the states and their ruling clans are still debated by scholars. The Mossi were in conflict with the Songhay Empire in the period from 1328 to 1333, and again between 1477 and 1498. In general terms, the Mossi were strong enough that they were never conquered until the French arrived in 1896-1897, but they were not strong enough to do more than raid the kingdoms along the Niger. Their expansion was by annexing other, often stateless, peoples at the edges of Mossi polities, peoples whose general culture was the same and whose languages were related. Within one generation of the French conquest, French writers had already employed the term mossification to describe the assimilationist expansion of the Mossi states into surrounding communities.