Native Americans are unique among domestic actors in that their relations with the U.S. government involve treaty making, with almost 600 such documents signed between the Revolutionary War and the turn of the twentieth century. We investigate the effect of constitutional changes to the treating process in 1871, by which Congress stripped the president of his ability to negotiate directly with tribes. We construct a comprehensive new data set by digitizing all of the treaties for systematic textual analysis. Employing scaling techniques validated with word-use information, we show that a single dimension characterizes the treaties as more or less “harsh” in land and resource cession terms. We find that specific institutional changes to treaty-making mechanisms had little effect on agreement outcomes. Rather, it is the relative bargaining power of the United States economically and militarily that contributes to worsening terms for Indians over the nineteenth century.