Sidenote: Hoxie calls his method an "ethnohistory of Indian policy," and he connects legal decisions, contemporaneous anthropology, popular fiction, representations of Indians at the World's Fair in 1893, and archival sources from government agencies and the Bureau of Indian Affairs into an interdisciplinary anthropological framework.
The first assimilation phase, the "Dawes era," started in 1879, hard on the heels of antislavery zeal and the failure of Reconstruction. Land policies were designed to slow settler assaults on Indian lands, and the campaign itself emphasized egalitarianism. Moves toward Indian citizenship and the development of an Indian education system indicated sincerity of the American "final promise" to compensate the victims of American expansion by giving them "full membership in a 'civilized' nation." However, this pledge only lasted a generation, and from 1900 to 1920 the rhetoric of assimilation was undermined by eased access to Indian lands and decreased funding for full assimilation. Rather than full citizens, Indians became nominal citizens as well as fully-regulated wards of the state.
Ironically, if the Dawes era assimilation policies had been aggressively pursued, Indian cultures would have been all but obliterated - though setting Indians spatially outside the United States and forcing them to retain their traditional ways destroys their culture as well. Hoxie doesn't pursue this tension, nor does he include many Native American voices or provide a particularly nuanced account of why, exactly, 1900 is THE year. He does, however, provide detailed discussion of assimilation policy throughout the whole period, and he shows how Native American policy became integrated into a larger project of statemaking in a newly diverse industrial society.