In the United States v. Third National Bank in Nashville et al., 1967, the U.S. Supreme Court ruled on a case involving antitrust laws and banking mergers. The Department of Justice sought to prevent two bank mergers in Nashville, Tennessee - one between Third National Bank and First City National, and another between Commerce Union Bank and American Commercial Bank - arguing that these would significantly reduce competition in violation of Section 7 of the Clayton Act which prohibits acquisitions that may lessen competition or create a monopoly. The banks argued they were exempt from this law due to provisions within the Bank Merger Act of 1966 which allowed such consolidations if approved by certain federal banking agencies after considering various factors including competitive effects. The Supreme Court held that while federal banking authorities could approve mergers under their statutory authority, this did not immunize them from antitrust scrutiny under Section 7 of the Clayton Act as both statutes must be given effect where possible. Therefore, even though a merger might have been approved by relevant banking authorities for reasons like financial stability or convenience to community etc., it could still violate antitrust laws if its impact lessened competition substantially.

In the dissenting opinion for United States v. Third National Bank in Nashville et al., Justice Harlan argued that the majority's decision was based on a misinterpretation of Section 7 of the Clayton Act, which he believed should not be applied to bank mergers. He contended that Congress did not intend for this section to apply to such cases when it enacted amendments in 1950 and again in 1966. Furthermore, he criticized the majority's reliance on competitive factors as an overriding consideration, arguing that other considerations like convenience and needs of communities served by banks were equally important under banking laws. He also disagreed with their interpretation regarding potential competition theory and its application here; instead suggesting a more nuanced approach would have been appropriate considering complexities involved in banking industry dynamics.