They're unarmed, but march like an army though the streets of this tiny country -- tens of thousands of black-clad men beating their chests in unison to a slow, mournful rhythm. With each violent thump of their fists, they recall the first in a long line of grievances Shia Muslims hold against their Sunni brethren.
Oblivious to the drizzling rain, an estimated 250,000 marchers, a third of all the people who live on this Persian Gulf island, hit themselves and cry the name of Imam Hussein, the Shia leader who fell under Sunni swords in the year 680.
In Bahrain, the annual Ashura festival of mourning is part religious ceremony, part demonstration of political power. With sectarian tensions high across the region, it's a reminder to Bahrain's elites that, although the Shiites are poor and disenfranchised, they dwarf the ruling Sunnis in number, and hold an angry grudge.
Situated across the Persian Gulf from Iran, Bahrain's Sunni rulers are as sensitive as anyone in the region to the apparent extraterritorial aspirations of Iran's ayatollahs and their influence on the kingdom's Shia majority. The precursors of today's royal family invaded from neighbouring Qatar in 1780 and drove out the Persians. But Iran didn't extinguish its claim to the island until 1970, and today, the Bahraini government still perceives an existential threat emanating from Tehran.
Iran is playing in many Middle East backyards, most notably in Iraq (through the various Shia political parties and militias that take their orders from Tehran), Lebanon (through the Iranian-funded Hezbollah movement) and the Palestinian territories (through Hamas, a Sunni group that turned to Iran for help when faced by a crippling Western boycott). But it's tiny Bahrain where Iran arguably wields the most influence, and where Tehran might first try to demonstrate its new clout if a showdown with the United States and its Arab allies becomes inevitable.
Bahrain's predominantly Shia opposition, which tried to stage an Iranian-style revolution in 1981, has been taking to the streets frequently in recent months, emboldened by events in Iraq, where a Shia majority has finally thrown off oppressive Sunni rule. But King Hamad Bin Isa al-Khalifa and his coterie believe the Persians still stand behind Bahrain's opposition and its calls for more democracy.
There's plenty of evidence here to support those fears. Photographs of Iran's supreme leader, Ayatollah Ali Khamenei, and his predecessor, Ayatollah Ruhollah Khomaini, adorn the walls of nearly every Shia home on the island. Portraits of Lebanon's Hassan Nasrallah and the yellow flags of Hezbollah are also popular. Both sides are well aware that with the fall of Saddam Hussein, Bahrain is the only remaining country where a Sunni minority governs a Shia majority.
"Wake up Sunnis!" reads one message that was sent to thousands of Bahraini cellphones ahead of recent elections fought along sectarian lines. "Don't be naive or your fate will be like the Iraqi Sunnis who lost their rights and their lives."
The execution of Mr. Hussein at the end of last year, viewed by Sunnis across the region as revenge-taking by a Shia lynch mob, provoked a second round of such messages, many of which accused Bahrain's Shiites of working for Iran.
The government regularly trots out the Iranian bogeyman to scare its allies in Washington into easing calls for democratic reforms. A more tangible Iran-headquartered group, calling itself the Islamic Front for the Liberation of Bahrain, bombed a hotel in Manama, the capital, in 1996.
Unnerved by Tehran's new swagger after the collapse of the U.S. plan for Iraq and the election of President Mahmoud Ahmadinejad, Bahrain recently joined five other Arab states in the Persian Gulf to announce that, like Tehran, they would explore the idea of a peaceful nuclear program. Egypt and Jordan, which are also predominantly Sunni and uncomfortable with Iran's growing assertiveness, have also announced they will look into their nuclear options.
Long-dormant tension between Sunnis and Shiites, unlocked by the violence in Iraq, is exploding throughout the Middle East, from the daily carnage in Baghdad to the dangerous sectarian standoff on the streets of Beirut. Animosity is also high in the Sunni-ruled kingdoms of Saudi Arabia and Kuwait, which have significant Shia minorities and the terrifying example of Iraq just across the border, and in Yemen, where clashes between government forces and Shia rebels have killed dozens of people in recent weeks. Even in countries such as Jordan and Egypt, which are overwhelmingly Sunni, governments have issued dire warnings about the dangers of a "Shia crescent" stretching from Bahrain and Iran, through Iraq and Shia-ruled Syria to Lebanon.