This paper proposes and tests an explanation as to why rational managers seeking to maximize shareholder value can pursue value-decreasing mergers. It can be optimal to overpay for a target firm and decrease shareholder value if the loss is less than in an alternative where the merger is undertaken by a product market rival. This paper presents a model based on synergies, market power and competition for merger targets. Consistent with the model the empirical results obtained here show a strong correlation between the returns of acquiring firms and close rivals around merger events. Keywords: acquisitions, auction, event study, oligopoly, preemption JEL classification numbers: G34, G14, D43, D44, L13