Monopolies were made illegal by the

I agree with the person above - with the passing of the Sherman Antitrust Act, monopolies were made illegal.
This act forbade any trusts, which included monopolies as well in the United States. The act was accepted by the government in 1890 while the president was Benjamin Harrison. Over time, the act was broadened to include other organizations as well, such as cartels. 

Monopolies were made illegal by the U.S. Congress due to the Sherman Antitrust Act