I concur with the other educator that Roosevelt filed lawsuits to dissolve more than 40 monopolies during his presidency. The United States saw more than 2,000 mergers between 1897 and 1901. The larger companies eliminated competition by selling their products below cost. As smaller companies went into bankruptcy, the larger companies dominated their respective markets. Theodore Roosevelt's presidency spanned the years 1901 to 1909. Although he laid the groundwork for trust-busting, many of the monopolies he targeted were actually dissolved during William Howard Taft's administration (1909-1913).
One of them was Standard Oil, which controlled 90% of the United States oil market. Through the Standard Oil Trust, J.D. Rockefeller controlled the refining, distribution, and marketing aspects of the oil industry. In 1911, the Supreme Court found Standard Oil guilty of violating anti-trust regulations, leading the monopoly to be broken up into 34 separate companies or "Baby Standards." Some of these companies were Standard Oil of New Jersey (which became today's ExxonMobil), Standard Oil of California (which became Chevron), Continental Oil Company (which became ConocoPhillips), and Standard Oil of Indiana (Amoco, which was later purchased by BP).
In 1911, the Supreme Court also ruled that the American Tobacco Company, the Durham Tobacco Company, and the R.J. Reynolds Tobacco Company had violated the 1890 Sherman Anti-Trust Act. The American Tobacco Company was formed through the mergers of five large cigarette manufacturers. The 1911 ruling was significant in that it broke up the cigarette monopolies two years after Theodore Roosevelt vacated the presidency.
The two most well-known trusts dissolved during Roosevelt's presidency were the ones involving Northern Securities Trust and the Beef Trust. The Beef Trust was made up of six leading meatpacking companies (Swift, Armour, Morris, Cudahy, Wilson and Schwartzchild), which controlled half of the American meat industry. The Supreme Court decided against the meat monopoly in 1905. The railroad and meatpacking industries were divested of their monopolies during Theodore Roosevelt's presidency, leading the way for other trusts to be broken up during Taft's presidency.