Three prominent anti-slavery activists and opposition politicians in Mauritania were sentenced to two years in prison earlier this month. Biram Dah Abeid, the runner-up in the 2014 presidential elections, was convicted on charges of “belonging to an illegal organization, leading an unauthorized rally, and violence against the police”—along with one of his aides Bilal Ramdane, and Djiby Sow, a civic and cultural rights campaigner. Abeid is the president and founder of the Initiative for the Resurgence of the Abolitionist Movement, the most prominent anti-slavery activist organization in Mauritania.

Mauritania straddles North Africa and sub-Saharan Africa where slavery has long been institutionalized along racial and cultural lines. The country is comprised of mainly three distinct groups including Bidan, Arab-Berbers who comprise about 30 percent of the population and hold the majority of wealth and political power. Bidan are typically slave owners; Haratine, a distinct group of African descent, are the slaves. Haratine are considered the lowest class regardless of an individual’s unofficial status as slave or free man. Haritine comprise around 40 percent of the population. “Afro-Mauritanians,” black Mauritanians who are not decedents of slaves and do not speak Arabic, comprise about 30 percent of the population.

Mauritania, officially abolishing slavery in 1981, was the last country in the world to do so. On paper, the 2003 Law Against Trafficking in Persons and Law 2007–048 guarantee that people are protected from human trafficking, but in reality the deeply rooted social structure based on slavery remains the main cause of human trafficking. Few slaveholders in the country have been convicted, and those who have are often released from prison only months later. While it is difficult to know the exact number of individuals enslaved in Mauritania, according to estimates, the country has the highest prevalence of human trafficking and slavery in the world, with 4 percent of the total population—roughly 155,600 people—enslaved. Mauritania is also listed on “Tier 3,” the worst ranking that a country can receive for human trafficking, in the U.S. Department of State’s “Trafficking in Persons” report. Tier 3 is reserved for “[c]ountries whose governments do not fully comply with the minimum standards and are not making significant efforts to do so.”

The conviction of Abeid, Ramdane, and Sow is not only a concern for the anti-slavery movement in Mauritania, but also for advocates of freedom of association and assembly. Abeid’s conviction in particular has implications not only for his work shedding light on the abhorrent institution of slavery in Mauritania, but for the development of a wider political opposition movement.

While Mauritania remains an important counterterrorism ally for the United States, the “informal” institution of slavery in Mauritania and Abeid’s conviction in particular should not go unnoticed by Congress and the Obama Administration. In September 2014, President Obama waived the legal assistance restrictions resulting from Mauritania’s Tier 3 ranking on the basis of of anti-trafficking efforts and U.S. national interests. While the justification may be necessary to promote U.S. interests in a challenging region, Abeid’s conviction and the current situation in Mauritania necessitate vigilant oversight on U.S. taxpayer assistance to Mauritania.