Reservations in what is now the United States were established for a combination of land realignment, sovereignty management, and removal of Native peoples from land desired by settlers. The core factors driving the creation and relocation to reservations include:

• land and resource expansion for settlers: as European and American settler expansion moved westward, the federal government sought to open more lands for settlement and development, often at the expense of Native homelands. This mindset helped justify relocating tribes to confined areas.
• treaties and legal mechanisms: numerous treaties and acts between the U.S. government and tribes designated specific lands as "reservations" where tribes would live and govern to some extent, but under federal oversight and with restricted rights to hunt, gather, or leave those lands freely when it conflicted with settler interests.
• removal policies and coercive legislation: the Indian Removal Act of 1830 and subsequent policies used force or pressure to relocate tribes from eastern territories to designated western areas, creating initial reservation systems. Later laws further shaped and sometimes narrowed tribal landholdings and governance.
• assimilation and administration goals: the government aimed to control Indigenous affairs through centralized agencies, regulate tribal life, and promote assimilation into Euro-American norms, often by restricting traditional powers and economic practices within defined reservation boundaries.

Key historical milestones often cited include:

• early treaties that ceded land and established smaller, controlled areas for tribes,
• the Indian Removal Act of 1830 enabling relocation to the West,
• the Indian Appropriations Act and subsequent laws that created and constrained reservations,
• the Dawes Act of 1887, which broke up communal tribal lands into individual parcels, accelerating land loss and undermining traditional sovereignty.

If you’d like, I can pull concise, sourced summaries from historical references and create a timeline highlighting the major acts and treaties that defined the reservation system.