Who broke up monopolies such as the northern securities company?

The breakup of monopolies like the Northern Securities Company was primarily driven by the U.S. government, particularly under President Theodore Roosevelt. In 1904, the Supreme Court ruled against the Northern Securities Company, a major railroad trust, citing the Sherman Antitrust Act. This decision marked a significant moment in antitrust enforcement, as Roosevelt's administration actively pursued trusts to promote competition and curb corporate monopolies.