"And every Indian born within the territorial limits of the United States to whom allotments shall have been made under the provisions of this act, or under any law or treaty, and every Indian born within the territorial limits of the United States who has voluntarily taken up, within said limits, his residence separate and apart from any tribe of Indians therein, and has adopted the habits of civilized life, is hereby declared to be a citizen of the United States, and is entitled to all the rights, privileges, and immunities of such citizens, whether said Indian has been or not, by birth or otherwise, a member of any tribe of Indians within the territorial limits of the United States without in any manner affecting the right of any such Indian to tribal or other property."—From the Dawes Act, 1887

What development did not help lead to the policy described in this excerpt?

Conflict with American Indians on the frontier
Failure of Reconstruction-era policies
Increased westward migration after the Civil War
Growth in land-grant colleges and homesteading