Final answer:
The U.S. government's policy of forced relocation of American Indian tribes was caused by the Indian Removal Act of 1830, leading to dispossessions like the Trail of Tears and subsequent confinement to reservations.
Step-by-step explanation:
The displacement of American Indian tribes during the westward expansion of the United States was primarily caused by the Indian Removal Act of 1830. This legislation authorized the forced relocation of Native tribes east of the Mississippi River to lands west of the river. The Cherokee resisted this policy, leading to the tragic Trail of Tears, but many tribes underwent forced removals including the infamous relocation to what is now Oklahoma. White settlers' encroachment on Native American lands and the desire to solve the "Indian problem" ultimately led to widespread dispossession and forced migrations, altering the Native Americans' way of life and causing immense suffering.
As more White Americans and European immigrants moved west after the Civil War, Native Americans were again displaced, this time to reservations, federal lands set aside where non-Indians could not settle. However, as White settlement expanded, these reservations became smaller and Native Americans experienced cultural and economic consequences due to repeated displacements.