How did the tactics of John D Rockefeller create a monopoly?

John D. Rockefeller employed aggressive tactics to create a monopoly in the oil industry, primarily through horizontal integration, which involved buying out competitors to control a significant share of the market. He also utilized vertical integration by controlling every aspect of oil production and distribution, from drilling to refining to transportation. Additionally, Rockefeller implemented secret deals and rebates with railroads, which allowed him to lower costs and undercut competitors. These strategies effectively eliminated competition and established the Standard Oil Company as a dominant force in the industry.