Before a law enforcement crackdown hobbled it in 2021, the Base established itself as one of the most active neo-Nazi accelerationist groups — a term for groups that seek to hasten societal collapse by violent means. Now the Base has rebuilt, to the extent that last year it earned a spot on the European Union list of sanctioned terrorist groups. A sudden burst of activity in Ukraine has renewed suspicions that the Base and its leader, Rinaldo Nazzaro, are linked to Russian intelligence and security services. “The Base’s activities in Ukraine suggest that there is more to this group than...