Unlike Section 7 of the Clayton Act, which permits the antitrust authorities either to restrain a merger beforehand or undo it thereafter, the Federal Reserve’s authority under the Bank Holding Company Act to disallow the merger on financial stability grounds appears to lapse after the merger is completed, but for the narrow authority vested in the FSOC.
Of course, one motivation for the passage of Hart-Scott-Rodino in 1976 was the perceived reluctance of courts to order potentially disruptive divestitures once companies had completed the merger.