On the night of April 15, 1869 a group of Algerians aligned with the French military, the Nememcha, raided a caravan primarily made up of Hammama, a rival group from the Tunisian side of the new colonial border. The raid cut down twenty-seven people, including children and women. The Nememcha were not initially punished. Only after an outcry raised by the father of an unrelated merchant killed in the raid did the Military Tribunal launch an investigation. Both sides claimed that hardships arising from the environmental calamities of the previous years – drought, locust invasion, epidemic disease – had heightened tensions and led to the confrontation. This “Affaire of the Oued-Mahouine,” as it came to be known, remained a touchstone in colonial politics. For instance, in 1899, in the French Chamber of Deputies, Mr. Marchal brought it up as proof that Muslim Algerians did not deserve full rights as French citizens. Through an examination of contemporary military correspondence, administrative reports, and newspaper coverage, this paper argues that environmental catastrophe combined with a new organization and expansion of state power to heighten frontier tensions and finally make concrete the colonial border between Algeria and Tunisia. The Affaire of the Oued-Mahouine had salience not only in the immediate political situation, but also in debates about rights and power in Algeria thirty years later. Study of this Affaire highlights the constructed and contested nature of colonial borders: the creation and policing of the new Algerian-Tunisian border calcified antagonistic relationships and shifted power relations in the region, determined legal status and protection for certain groups over others, and brought new territories and tactics into the realm of colonial state action. The environmental disaster of the preceding years only exacerbated these processes, as state institutions such as the military and police expanded their reach and adapted new tactics in response to these crises. This expanded state apparatus redefined Algerian and Tunisian subjectivity in ways that contributed to the creation of this important Affaire.