Final answer:
John D. Rockefeller was able to make a lot of money during the Civil War due to his control over the oil industry, manipulation of the market, and low production costs.
Step-by-step explanation:
During the Civil War, John D. Rockefeller was able to make so much money largely due to his control over the oil industry. By running his competitors out of business and controlling most of the oil refineries in Cleveland, Rockefeller was able to manipulate the market and lower prices. His secret deals with suppliers and railroads further allowed him to drive down costs and force competitors to sell their refineries to his Standard Oil Company. Additionally, the demand for oil during this time was mainly for lubricants and fuel for heating lamps, which were relatively easy to produce from crude oil.