Transactions in today's highly globalized world are often subject to parallel investigations in different jurisdictions. These parallel reviews create the possibility of divergent, or even conflicting, outcomes. But such divergent outcomes have historically been rare, as U.S. and European Commission (EC) antitrust agencies have collaborated to formulate "an advisory framework for interagency cooperation" in merger investigations. See Best Practices on Cooperation in Merger Investigations, US-EU Merger Working Group. Similarly, U.S. Federal Trade Commission (FTC) and EC regulators have committed to work together through the EU-U.S. Joint Technology Competition Policy Dialogue. And EC and United Kingdom regulatory leaders have called for a formal cooperation agreement between their respective agencies.

Notwithstanding pledges of cooperation, since Brexit at the end of 2020, the U.K. Competition and Markets Authority (CMA) anticipates that a significant proportion of its merger reviews will run in parallel with investigations by the EC, raising concerns that the incidence of divergence will increase. The recent merger review decisions by the CMA and the EC regarding Microsoft's proposed acquisition of Activision-Blizzard have further thrust the topic of divergence into the spotlight. This article explores possible reasons why global antitrust agencies have diverged in their past merger investigation decisions, trends regarding divergence in recent years, and the implications of these trends on merger control moving forward.