Who is famous for vowing, " I have not yet begun to fight."

John Paul Jones.

Some extra info I wasn't able to put in the comments;

John Paul Jones was the United States' first well-known naval commander in the American Revolutionary War. His reputation was gained from his decisions during said war in British territory. He is also referred to as "Father of the American Navy". As for his famous words, "I have not yet begun to fight", they were said, and born, during a battle on British waters, in which the rival ship had told him and his men to surrender. In response to these words, the quote was created. He kept to his vow, as he'd fought bravely, and won the battle.