The story: The Islamic State group (IS) has claimed responsibility for a rare attack on a Shiite mosque in Oman. The shooting, east of Muscat, left Indian and Pakistani nationals dead and scores injured. The incident is raising concern that IS may be establishing a foothold in a stable corner of the Arabian Peninsula known for its religious tolerance and sectarian harmony.

The coverage: The assailants targeted Shiite Muslims who were marking the annual religious occasion of Ashura at the Imam Ali bin Abi Talib mosque in...