In the case of United States v. Philadelphia National Bank et al., 1962, the U.S. Supreme Court ruled against a proposed merger between two large commercial banks in Philadelphia - The Philadelphia National Bank and Girard Trust Corn Exchange Bank. The Department of Justice argued that this merger would significantly reduce competition in commercial banking within the four-county area surrounding Philadelphia, violating Section 7 of the Clayton Act which prohibits mergers and acquisitions where their effect may be to lessen competition or create a monopoly. The court agreed with this argument, stating that any significant increase in market concentration caused by a merger was presumed to be illegal under Section 7 unless it could be justified on other grounds such as improving efficiency or saving a failing company – neither of which were applicable here. This decision marked an important precedent for future cases involving bank mergers and antitrust law.

In the dissenting opinion for United States v. Philadelphia National Bank et al., Justice Harlan argued that the majority's decision to block a merger between two banks based on potential anti-competitive effects was misguided. He contended that the Clayton Act, which prohibits mergers and acquisitions where the effect may be substantially to lessen competition or tend to create a monopoly, should not apply in this case because banking is heavily regulated by other laws specifically designed to prevent undue concentration of power. Furthermore, he disagreed with how the majority defined "line of commerce" and "section of country," arguing their interpretation was too broad and inconsistent with previous cases. Lastly, he criticized their reliance on statistical data as evidence of anticompetitive effects without considering other factors such as market dynamics or regulatory oversight.