As a reaction to the violent clashes, both governments decided to expel each other’s citizens and to close the border. Attempts by members of the Mauritanian elite to challenge the land rights of local Mauritians and expel them to Senegal in order to derive benefit from the new dams further complicated the situation.
In April 1989, Fulani herdsmen and Mauritanian Soninke famers clashed over grazing rights in the Senegal River Valley, which demarcates the Mauritanian-Senegalese border. Mauritanian border guards intervened, killing two Senegalese peasants and taking several prisoners. As a result riots broke out in Senegal, targeting the Mauritanian immigrant population, which was quite numerous on the southern Senegal River bank. Subsequently, Senegalese immigrants were victims of reprisal attacks in Nouakchott and other Mauritanian cities. Both countries began expelling Mauritanian and Senegalese nationals and the Mauritanian-Senegalese border was closed. By the end of April several hundred were killed or injured and several thousand displaced (UCDP, 2014). Mauritanian-Senegalese diplomatic relationships remained strained until the reopening of the border in 1992.
While the incident that sparked the violence centred on a local land use conflict between farmers and herders, the wider conflict opposing Mauritanians and Senegalese has to be understood against the background of on-going racism in Mauretania. There has been a significant north/south divide in the country, roughly corresponding to a divide between the ‘black’ and ‘white’ populations. Frequently, villages inhabited by ‘black’ Mauritanians living close to the southern border to Senegal were evacuated, the inhabitants stripped of their identification and deported to Senegal or Mali. This discrimination of southern ‘black’ Mauritanians created a tense situation not only in Mauretania but also in Senegal, where numerous ‘white’ Mauritanian immigrants had opened businesses (UCDP, 2014).
A further factor contributing to these tensions were plans to build two large dams on the Senegal River and the Bafing River tributary in Mali. The dams were supposed to regulate the river's flow, while producing hydropower and allowing the expansion of irrigated agriculture in response to droughts and food shortages, which had been particularly severe in the 1970’s and 1980’s. Speculations about the increased value of land where irrigated agriculture would become possible, however, encouraged the predominantly ‘white’ Mauritanian elite to alter land legislation in order to strip ‘black’ Mauritians in the river valley of their rights and expel them to Senegal. This contributed to grievances against white Mauritanians in Senegal (Homer-Dixon, 1994).
The Mauritanian-Senegalese border was closed and diplomatic relations between the two countries ceased on 21 August 1989. Mediation attempts by the Organization of African Unity (OAU) in 1990 were not successful. Finally, Senegal's President Abdou Diouf managed to work out an agreement with his Mauritanian homologue Maaouya Ould Sid'Ahmed Taya and a treaty was signed by the two countries on July 18, 1991. The border was reopened on 2 May 1992 and the repatriation of refugees began, albeit slowly (UCDP, 2014; Onwar project, 2014).