The UK is reportedly planning to officially designate the Russian mercenary firm Wagner Group as a terrorist organisation. This would put Wagner on a list with 78 other groups, including ISIS, Al Qaeda and newer white supremacist organisations. The UK has had lists of “proscribed organisations” for decades, previously including groups in the Northern Ireland conflict, and then creating a more global list in the early 2000s. Once an organisation is listed, it becomes a criminal offence to belong to the group or support the group — with a punishment of up to 14 years in prison. Dozens of other...