I'm trying to find out the name of a book a read in high school(1984) about wars fought in an arena(sci-fi)

It was a futuristic story of a young man who was selected to represent the USA in battle. The country with the last man alive won whatever the battle was about. The last fighter was granted immunity from any crime committed in the past and futre. The story was in a book of short stories and it was probably from the late 70's to early 80's. Please help!!!