Heightened ethnic tensions were the catalyst for the Mauritania–Senegal Border War, which started as a result of a conflict in Diawara between Moorish Mauritanian herders and Senegalese farmers over grazing rights. On 9 April 1989, Mauritanian guards killed two Senegalese. Following the incident, several riots erupted in Bakel, Dakar and other towns in Senegal, directed against the mainly Arabized Mauritanians who dominated the local retail business. The rioting, adding to already existing tensions, led to a campaign of terror against black Mauritanians, who are often seen as 'Senegalese' by Bidha'an, regardless of their nationality. As low scale conflict with Senegal continued into 1990/91, the Mauritanian government engaged in or encouraged acts of violence and seizures of property directed against the Halpularen ethnic group. The tension culminated in an international airlift agreed to by Senegal and Mauritania under international pressure to prevent further violence. The Mauritanian Government expelled tens of thousands of black Mauritanians. Most of these so-called 'Senegalese' had no ties to Senegal, and many have been repatriated from Senegal and Mali after 2007. The exact number of expulsions is not known but the United Nations High Commissioner for Refugees (UNHCR) estimates that, as of June 1991, 52,995 Mauritanian refugees were living in Senegal and at least 13,000 in Mali. From November 1990 to February 1991, between 200 and 600 (depending on the sources) Fula and Soninke soldiers and/or political prisoners were executed or tortured to death by Mauritanian government forces. They were among 3,000 to 5,000 blackspredominantly soldiers and civil servantsarrested between October 1990 and mid-January 1991. Some Mauritanian exiles believe that the number was as high as 5,000 on the basis of alleged involvement in an attempt to overthrow the government. The government initiated a military investigation but never released the results. In order to guarantee immunity for those responsible and to block any attempts at accountability for past abuses, the Parliament declared an amnesty in June 1993 covering all crimes committed by the armed forces, security forces as well as civilians, between April 1989 and April 1992. The government offered compensation to families of victims, which a few accepted in lieu of settlement. Despite this amnesty, some Mauritanians have denounced the involvement of the government in the arrests and killings. In the late 1980s, Ould Taya had established close co-operation with Iraq, and pursued a strongly Arab nationalist line.