4. Jack Dempsey
A cultural icon, Jack Dempsey was the heavyweight champion of the world from 1919 to 1926. His aggressive fighting style and ferocious punching power made him one of the most popular boxers in history. Furthermore, he was so popular that his fight against George Carpentier was the first $1 million gate.
Dempsey served in the US National Guard from 1942. When World War 1 went down, fans criticized him for not enlisting. But he bowed to the pressure and enlisted. As a result, he was present on the USS Wakefield for the invasion of Okinawa in 1945. The navy released him from active duty the following year, but he remained in reserve until 1952.