SAUDI Arabia has seemed more or less sheltered from the wars and sectarian bloodletting that have raged all around it in Iraq, Syria and Yemen. No longer.

On May 22nd a suicide bomber struck a mosque in the eastern region of Qatif, which is home to the majority of the kingdom’s 2.8m Shias (some 10% of the population). At least 20 people were killed as they gathered in the Imam Ali Mosque; scores more were injured. Islamic State (IS), which has long threatened to attack Shiites in the kingdom, claimed responsibility. If true, it signals that the sectarian violence raging to Saudi Arabia's north and south is now penetrating the kingdom itself, which IS now calls “Najd Province”.

Fresh from capturing the town of Ramadi in Iraq, and the town of Palmyra in Syria (home to some of the world’s finest ancient ruins), the bombing in Saudi Arabia strengthens the perception that the IS “caliphate” is again on an expansionary march. Hitherto IS has had no overt presence in the kingdom, but has made it clear it sees it as a target. In November, a recording by a man purporting to be Abu Bakr al-Baghdadi, IS’s chief, called on followers to “erupt volcanoes of jihad everywhere”, and announced the extension of his “state” to Saudi Arabia, among other places.

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The suicide bomb on the mosque is the most deadly of a spate of recent attacks against the extremists’ favourite targets: Shia, security forces and Westerners. In November a handful of Shia were killed by gunmen during celebrations of Ashura, an annual Shia rite of mourning. American, Danish and Canadian expatriates have been shot, one fatally. In April the Saudi government increased security after receiving threats to oil installations and shopping centres.

The attack on the mosque will further inflame sectarian tensions, which have already risen sharply as a result of the war Saudi Arabia’s new king, Salman, is waging against the Houthis, an armed Shia movement in neighbouring Yemen. Iran has denounced the Saudis almost daily, and several Saudis have been killed in retaliatory cross-border attacks, most recently on May 21st.

“The war on Yemen is rebounding on Shias at home,” said a Shia activist in London. “We are becoming the soft-target.” As families begin burying their dead, Shia protests are expected to grow.

The kingdom is better-placed than most countries to deal with security threats at home. Muhammad bin Nayef, the interior minister and recently-appointed heir to the throne, crushed a bombing campaign by al-Qaeda in the 2000s and has cooperated closely with the West ever since. The country runs a rehabilitation programme in which graduates are given a wife and a chunk of cash. It is said to have invested millions of dollars in technology to monitor trouble-makers—which the Saudis define worryingly broadly to include citizens calling for more freedom of expression. The government has reportedly arrested nearly 100 suspected members of IS since December and foiled several terror plots.