Lieutenant Colonel George Armstrong Custer led an expedition into the Black Hills — territory that was formally part of the Great Sioux Reservation under the 1868 treaty, explicitly protected from American intrusion.

The expedition’s stated purpose was partly military reconnaissance, partly scientific survey.

One of its actual purposes was to assess whether the Black Hills contained gold.

They did.

The discovery was announced publicly in August 1874.

By the following year, thousands of prospectors were moving into the Black Hills in direct violation of the 1868 treaty.

The U.S. government attempted halfheartedly to enforce the treaty provisions by turning back intruders, but the flood of prospectors was too large and the government’s commitment to protecting Lakota rights too limited for the attempt to succeed.

Negotiations to purchase the Black Hills from the Lakota failed.