The early reservation years of the late nineteenth century comprise a period of intellectual mobility and activism for Native Americans. This article argues that the thousands of letters written by western Native Americans to non-Natives during this period were a consequential weapon against U.S. colonialism. By highlighting the history of Wolf Chief, a determined Hidatsa activist, this article reveals the many strategies of Native Americans who wrote to U.S. presidents, officials, newspapers, and others to decolonize their lives, seek justice, counteract colonial abuses, and preserve a sense of self-determination. U.S. policy makers hoped that a more literate Native population would be transformed, that educated Indians might understand the virtues of Americanization, and that literacy might quell opposition to U.S authority. Instead, Native Americans used the written word in the early reservation years to challenge a massive colonial apparatus in ways that scholars have not recognized. Wolf Chief’s advocacy from the Fort Berthold Reservation was rooted in the conviction that the Three Tribes deserved a meaningful role in shaping their own futures and protecting their autonomy. His wide-ranging correspondence emphasized the importance of including Native perspectives in the management of reservation affairs, as he believed that effective solutions required the active involvement of his people.