Why did relations between Mauritania and Senegal deteriorate in 1989?

The deterioration of relations between Mauritania and Senegal in 1989 was driven by a combination of long-standing ethnic tensions and economic competition. Specifically, the conflict was fueled by disputes over herding rights along the Sénégal River and deep-seated resentment regarding the roles citizens of each country played in the other's local economy.1

These tensions escalated rapidly in 1989, leading both nations to expel tens of thousands of each other's citizens and sever diplomatic ties, bringing the two countries to the brink of war. Diplomatic relations were not restored until April 1992, and the process of repatriating the large refugee populations displaced by the conflict remained incomplete well into the early 2000s.1