Conversation so far:
Who began The Great Sioux War of 1876?
By the early 19th century, the Northern Cheyenne became the first to wage tribal-level warfare.
Who was Sitting Bull?
Sitting Bull in Standard Lakota orthography, also nicknamed Húŋkešni  or Slow) was a Hunkpapa Lakota leader who led his people during years of resistance to United States government policies.
What did Sitting Bull's band of Hunkpapa do preceding The Great Sioux War of 1876?
Sitting Bull's band of Hunkpapa continued to attack migrating parties and forts in the late 1860s.

Question: When was this?
Self-contained rewrite: When was Sitting Bull's attacks on the Northern Pacific Railway?