The United States v. United States Steel Corporation et al., 1919, was a landmark case in which the U.S government sued the U.S Steel Corporation under antitrust laws for monopolizing and restraining trade in steel manufacturing and sales. The government argued that the corporation's size alone constituted an illegal restraint on trade, regardless of its behavior or intent. However, the Supreme Court ruled against this argument stating that size does not determine guilt under anti-trust laws; rather it is about whether there is an unreasonable restraint on competition due to monopoly power being used maliciously. In other words, just because a company has achieved monopoly status doesn't mean they are automatically guilty of violating antitrust law - their actions must also be considered harmful to competition.

In the dissenting opinion for United States v. United States Steel Corporation, Justice Louis Brandeis argued that the majority's decision to not break up U.S. Steel was a mistake and would have long-term negative consequences on competition in American industry. He believed that U.S. Steel had indeed violated antitrust laws by suppressing competition through its control of supply and pricing in the steel market, which he saw as harmful monopolistic practices detrimental to free enterprise system. Furthermore, he disagreed with the majority's view that size alone did not constitute monopoly; instead, he asserted that large corporations like U.S. Steel inherently posed threats to competitive markets due their ability to manipulate prices and stifle smaller competitors.