Complex sentence: A combined force of Santee and Teton Sioux forces had been defeated at the battle of Big Mound, the Native forces fled that battlefield and were chased endlessly by U.S. cavalry.
Split into simpler sentences: A combined force of Santee and Teton Sioux forces had been defeated at the battle of Big Mound. They fled that battlefield and were chased endlessly by U.S. cavalry.