How did Anericans build fortunes in the oil and steel industries?

Americans, such as Rockefeller and carnegie, built fortunes in the oil and steel industries using vertical and horizontal integration, forming trusts, lowering prices, making it impossible for smaller companies to compete, forming corporations, setting up stocks, and basically monopolizing the industry. They also offered rebates to customers, pressuring customers not to deal with other companies.

Andrew Carnegie in steel and John D. Rockefeller in oil industry built fortunes by buying the competition, thus creating monopolies that could charge prices much higher than costs and earn large profits.