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The past 40 years have witnessed a transformation in horizontal merger enforcement in the United States. The weight given to market concentration by the federal courts and by the federal antitrust agencies has declined dramatically, and increasing weight has been given to three arguments often made by merging firms in their defense: entry, expansion, and efficiencies. This paper documents this shift and provides examples where courts have approved highly concentrating mergers based on limited evidence of entry and expansion. It shows that the decline in antitrust enforcement is ongoing, especially at the current Justice Department. It argues in favor of reinvigorating horizontal merger enforcement by partially restoring the presumption that mergers that produce high combined market shares are questionable. It proposes several routes by which the government can establish its prima facie case, distinguishing between cases involving coordinated versus unilateral anticompetitive effects.