Complex sentence: Hopkins had been a famous distance rider, a cowboy and a dispatch rider for the United States government; in the latter capacity he carried a message to the U.S.7th Cavalry Regiment authorizing the Wounded Knee Massacre of Lakota Sioux.
Split into simpler sentences: Hopkins had been a cowboy and a dispatch rider for the United States government. In this capacity, he had carried a message to the U.S.7th Cavalry Regiment, authorizing the attack at Wounded Knee that resulted in massacre of a band of Lakota Sioux.