Which act made it illegal to buy up companies in the same industry to gain a competitive advantage?

The act that made it illegal to buy up companies in the same industry to gain a competitive advantage is the Clayton Antitrust Act of 1914. This legislation aimed to prevent anti-competitive practices and monopolistic behavior, particularly focusing on Mergers and Acquisitions that could substantially lessen competition. It built upon earlier antitrust laws, such as the Sherman Antitrust Act of 1890, by addressing specific practices that could harm market competition.