Qatar’s decision to leave Opec has seismic ramifications for the future of the organization. Its crude output may be modest, but Qatar was one of the longest-running members. The move shatters Opec’s proud claim that the group can overcome the often-fractious politics between its members.

Qatar is to leave Opec effective 1 January 2019 in a move that makes the widening fissures running through the group impossible to hide. Members have long pointed proudly to the group’s...