Abstract: This dissertation uses primary source documents, linguistic analysis, and secondary sources to closely examine tribal leadership in the Confederated Tribes of the Umatilla Indian Reservation and the Confederated Bands and Tribes of the Yakama Indian Reservation from 1854 to 1940. I aim to examine sovereignty in the terms and with the goals that tribal leaders in 1854-1855 used, and carry that definition forward through leadership and activism on both reservations. Plateau tribal leaders, I argue, defined sovereignty as a web of reciprocal relationships between people and land, in which people had rights – but so did land, and so did the other-than-human relatives who also resided on the Plateau. Although the context on and around the Plateau changed politically from 1855 to 1940, Plateau tribes adapted their strategies to that context without changing their core goals. The first decades of American settler-colonialism on the Columbia Plateau, from the 1840s to the 1880s, were catastrophically violent. Leaders of the Cayuse, Umatilla, Walla Walla, and Yakama bands adopted multiple strategies, sometimes at odds with one another, to survive. Between the 1870s and 1910s, however, it is clear that band leaders worked together to protect each other, their land, and their people from American violence. Over the early 20th century, more grassroots activism becomes clear, particularly in labor, religion, and education. The determination of Plateau leaders and people to maintain their sovereign relationships shaped their political context at least as much as Federal Indian policy did, by the middle of the 20th century.