Complex sentence: He served as a captain under General George Armstrong Custer in the Indian Territory in present-day Oklahoma, where he died at the age of 24 while leading a charge in the Battle of Washita River.
Split into simpler sentences: He served as a captain under General George Armstrong Custer in the Indian Territory in present-day Oklahoma. He died at the age of 24, while leading a charge in the Battle of Washita River.