The island nation of Bahrain is allied with the U.S. and Saudi Arabia; it is also a Shiite-majority country governed by a narrow Sunni elite. Through 2011, restive Shiites periodically rioted or engaged in low-grade terror: throwing Molotov cocktails and setting off bombs meant to frighten rather than to kill or maim. But since then, the armed opposition has grown more sophisticated, relying on small, harder to penetrate cells, carefully planned attacks, and far more sophisticated and deadly explosives. Michael Knights and Matthew Levitt see much evidence of support from Iran and Hizballah:
Since 2011, Iran and its proxy militias in Lebanon and Iraq have mounted an unprecedented effort to train, activate, and resupply cells ready to set off improvised explosive devices (IEDs) inside Bahrain. Following the military suppression of Arab Spring protests in Bahrain, a number of Shiite youths traveled abroad to receive Iranian training in camps and battlefronts in Iran, Lebanon, Iraq, and Syria. Iran’s effort brought significant quantities of military high explosives into Bahrain and assisted Bahraini cells in developing workshops capable of churning out reliable, remote-controlled IEDs. Bahraini militants [also] have emerged as a smaller, tempered movement with better operational security.
Knights and Levitt suggest that Tehran may be waiting for general unrest, as during the 2011 Arab Spring, to activate the various cells it is cultivating in Bahrain—the same strategy it employed to foment and exploit chaos in Lebanon, Iraq, Syria, and Yemen. Or the plan could be to use Bahrain as a springboard to Saudi Arabia:
Located across a 25-kilometer causeway from Bahrain, the Eastern Province of Saudi Arabia is a predominantly Shiite area that holds more than 20 percent of the world’s total proven oil reserves and serves as the center of the kingdom’s oil and petrochemicals industries. The Shiite population has become more restive in recent years. . . .
The [recent] increase in violence in Saudi Arabia’s Eastern Province is even more notable than in Bahrain. In 2014-2015, there were just four attacks on security forces, all shooting incidents that left a total of five officers dead and three wounded. In 2016-2017, the number of attacks jumped to 24, with 18 killed and 39 wounded, with an even balance of shootings and bomb attacks. The importation of IEDs from Bahrain may be one factor in this change. . . .
On November 10, 2017, Bahraini militants may have acted on their intent [to bring their operations to Saudi Arabia] by bombing a key pipeline . . . supplying Saudi Arabian crude to the Bahrain Petroleum Company refinery.
Read more on Washington Institute for Near East Policy: http://www.washingtoninstitute.org/policy-analysis/view/the-evolution-of-shia-insurgency-in-bahrain