When he was campaigning for Brexit in 2016, Boris Johnson always insisted that Britain’s departure from the European Union would be a straightforward and friendly process between longstanding allies. Fast-forward four years, Johnson is now prime minister—and the EU is now taking legal action against his government for breaking the international treaty that was the foundation for Britain’s withdrawal from the bloc. The British prime minister wants to unilaterally change some of the terms of the agreement that he signed last year with European leaders. Ursula von der Leyen, the European Commission president, said that would breach the terms of the treaty. She’s given Johnson a month to change course, after which there could be a dramatic showdown at the EU’s top court.