This article explores the intertwined history of late 19th century Sudan and Ethiopia from a transboundary perspective. Focusing on a specific border zone in a period of growing tensions between the newly-established Mahdist state and the expanding Christian kingdom (1885-1889), the study analyzes how various patterns of commercial, military and diplomatic interactions shaped and were shaped by the Sudanese-Ethiopian conflict. That the ruling elites used religious arguments to legitimize military operations did not prevent intense flows of people, things and representations in the borderlands northwest of Lake Tana. A precise historical enquiry into the changing conceptualizations of the border, trade dynamics in goods and human beings, war booty practices, and diplomatic epistolary exchanges allows assessing the complex role of this portion of the border zone in the evolution of Sudanese-Ethiopian relations. The research is based on previously untapped Mahdist archives, European travel accounts, and scholarly works in Arabic, English, French and German. It is meant to contribute to the rapidly expanding field of border studies, as well as give new insights into the entangled history of African societies which retained their political sovereignty in the early years of the European scramble for Africa.