How did John D. Rockefeller make his

principal fortune?

Answer: Petroleum.

Explanation: John D. Rockefeller built his first refinery in 1870. He had a monopoly of oil in the oil industry of the U. S.

He became rich buying the companies of his rivals and creating companies to sell his products in the world. In 1882 he had the control of 90% of the refineries of the country. He was considered a Captain of Industry, for making so much money and for become a philanthropist, that donated a lot of money.