Hundreds of Saudis have gone to Iraq and Syria to join IS despite a government ban on supporting the terrorist group. This exile community provides IS with a cadre of potential suicide bombers and links to family members and associates back at home.

By attacking Shiite mosques and other soft targets, IS hopes to fan sectarian tensions and exploit Sunni hatred for Shiites to build a cadre inside the kingdom. The Saudi family has condemned the attacks and promised to destroy IS cells inside the country. King Salman bin Abdul-Aziz Al Saud has promised swift punishment.

Caliph Ibrahim, or Abu Bakr al-Baghdadi, has publicly called for the downfall of the House of Saud. IS claims it has already established an underground infrastructure in what it calls the "vilayet of the Nejd." Saudi authorities have arrested dozens of Saudis accused of belonging to IS cells.

The first IS terrorist attack on Saudi Shiites took place in November. This month there have been two suicide attacks on Shiite mosques on successive Fridays. The first killed 21 worshippers inside a mosque, the second bomber was stopped outside the mosque before blowing himself up, killing four.

The Islamic State (IS) is targeting the Sunni-Shiite sectarian divide in Saudi Arabia to create discord and undermine the royal family's rule. The kingdom is vulnerable to sectarian strife given the family's intimate connections to the Wahhabi Sunni clerical establishment, years of suppression of Shiite Saudis and the war in Yemen .

Shiites compose between 10-15% of the Saudi population, or roughly 3 million people. They reside in two main areas: Eastern Province along the Persian Gulf (home to the kingdom's oil wealth) and the southwest region along the Yemeni border. They are not a homogenous community; different strands of Shiite beliefs are present. So far, all the IS operations have been in Eastern Province.

Eastern Province has experienced violent unrest for decades, ever since the Iranian revolution in 1979. The Arab Spring led to an upsurge in protests and demonstrations calling for an end to discrimination against Shiites in the kingdom. The governor, Prince Saud bin Nayef, is the older brother of Crown Prince Mohammed bin Nayef. Both have well-established reputations for their tough responses to protests and terrorism.

Decades of discrimination and repression undermine government attempts to show support and sympathy for the community now that it is under attack from IS fanatics. Too often, legitimate Shiite protests have been labeled Iranian subversion by Riyadh in the past. Iran openly expresses sympathy for the Shiites in the kingdom and has backed anti-regime violence. Iran and Hezbollah were involved in the 1996 attack on the US Khobar barracks that killed 19 American airmen in Eastern Province and injured hundreds of Saudis.

The Wahhabi faith believes Shiism is a deviation from Islam and has treated Shiites as infidels for 2½ centuries. The late King Abdullah tried to ease Sunni-Shiite tensions in his reign and to encourage greater tolerance but with only limited success. Prince Saud bin Nayef worked closely with Abdullah on this effort. Hard-core Sunni propaganda against Iran and Shiites is widespread in Saudi Arabia, often blessed by senior clerics.

The war in Yemen adds to the combustible mix. The Saudi campaign to oust the Houthi Zaydi Shiite rebels from Sanaa and other Yemeni cities has an inherent sectarian element. By accusing the Zaydis of being Persian pawns, the Saudis and their allies play to underlying sectarian passions. Pakistan, which has its own deep sectarian problem, warned the Saudis that the war in Yemen would fan sectarian tensions across the Islamic world.

The Saudis have a large counterterrorist and internal security community led by Crown Prince Nayef with a track record of getting results. The Ministry of the Interior and the Saudi National Guard have an extensive presence in Eastern Province and the southwest. But repressing anti-Shiite violence while curbing Shiite protests in the midst of an external war against a Shiite enemy backed by Iran is a delicate and complex task that will test the crown prince and National Guard commander Prince Mitab bin Abdullah.

For its part, IS will probably try to expand its sectarian war into Saudi Arabia's allies Bahrain and Kuwait. If the caliphate can attack Shiite targets in Bahrain and Kuwait, the wave of violence will spread even further into Arabia. The caliphate is ambitious and searching for more victims.