Complex sentence: During the Spanish-American War he served as a Captain of the 6th US Volunteer Infantry, also known as the Sixth Immunes, which was mustered at Knoxville, Tennessee and saw service in Puerto Rico.
Split into simpler sentences: During the Spanish-American War he served as a Captain of the 6th US Volunteer Infantry. The 6th US Volunteer Infantry, also known as the Sixth Immunes, was mustered in at Knoxville, Tennessee and saw service in Puerto Rico.