Indian Appropriations Act, 1871

...hereafter no Indian nation or tribe within the territory of the United States shall be acknowledged or recognized as an independent nation, tribe, or power with whom the United States may contract by treaty.
...nothing herein contained shall be construed to invalidate or impair the obligation of any treaty heretofore lawfully made...

Treaties signed before the act were still valid, but all future negotiations were replaced with bilateral agreements between tribes and Congress.

Indian Appropriations Act 1871

The Indian Appropriations Act launched a new era in Federal-tribal relations by ending the practice of forming treaties with tribes as sovereign nations. Native Americans protested the policy change.

National Archives, General Records of the U.S. Government