Answer:
allowing powerful monopolies to dominate interstate commerce
Explanation:
In late 1800s, United States faced massive problem in its economy. At the time, many corporations conducted unethical businesses practices that make them become the sole producer of a certain product and working together in a price-fixing scheme.
This make the common people have to pay their products with a significantly higher price while having to survive with low wages & terrible standard of living. The government managed to put an end to this practice by passing the Sherman Antitrust Act on July 2 1890.
In his speech above, John Marshall Harlan was warning the people about this. He didn't want them to forget what happened if the country let powerful monopolies to dominate interstate commerce.