Complex sentence: Though Davis was highly unpopular among former Confederates, and most material written about him for many years was unfavorable, he was considered to have been a hero for the Union Army and gained the respect and friendship of Spanish-speaking residents on the Rio Grande frontier.
Split into simpler sentences: Though Davis was highly unpopular among former Confederates, and most material written about him for many years was unfavorable, he was considered to have been a hero for the Union Army. He also gained the respect and friendship of Spanish-speaking residents on the Rio Grande frontier.