RIFFA, Bahrain — Black and yellow concrete barricades block the roads entering a wealthy Sunni
enclave, where foreign-born Sunni soldiers in armored personnel carriers guard the mansions of the
ruling family and the business elite.

   Beyond the enclave are impoverished villages of Shiites, about 70 percent of
Bahrain’s more than 650,000 citizens, where the police skirmish nightly with young men wielding
rocks and, increasingly, improvised weapons such as homemade guns that use fire extinguishers to
shoot rebar.

   Their battles are an extension of sectarian hostilities nearly as old as Islam.
But they also are a manifestation of a radically new scramble for power playing out across the
region in the aftermath of the U.S. invasion of Iraq and the Arab Spring revolts.

   This island nation off the eastern coast of Saudi Arabia was the first place
where Arab Spring demands for equal citizenship and democratic governance degenerated into a
sectarian feud, and at first it seemed to be an anomaly. But Bahrain’s experience now appears to
have been a harbinger of what was to come as centuries-old but newly inflamed rivalries between
Sunni and Shiite Muslims tear apart much of the region — threatening to erase the borders of states
such as Syria and Iraq, destabilizing Bahrain and Lebanon, and accelerating a regional contest for
power and influence between Shiite Iran and Sunni Saudi Arabia.

   Scholars and activists say the sectarian violence gripping the Middle East is
not simply the unleashing of religious rivalries once suppressed by the secular autocrats who ruled
the region. Instead, they say, the religious resentments have been revived and exploited in a very
earthly power struggle.

   “There are forces that keep the tension alive in order to get a bigger piece of
the cake,” said Sheikh Maytham al-Salman, a Shiite scholar who was detained for nine months and
tortured by the Bahraini police in 2011 because of his support for the uprising.

   Pearl Square, where demonstrators staged a weeks-long sit-in three years ago,
has been turned into a permanent military camp. Its namesake statue has been demolished in a grim
memorial of the day in March 2011 when vehicles and troops from the neighboring Sunni monarchies
rolled across the causeway from Saudi Arabia to crush the Shiite-dominated movement for democracy.

   Once aroused, however, sectarian wrath can be unpredictable and hard to control,
even boomeranging against those who might have sought to exploit it. From the first stirring of
Arab Spring protest in Syria, for example, the government of President Bashar Assad and his Iranian
backers sought to portray the movement as a sectarian power grab by certain Sunni extremists, in
order to rally Christians and other religious minorities against it. Meanwhile, Saudi Arabia and
other Sunni-led Persian Gulf states sponsored satellite broadcasts firing up Sunni resentment of
Shiite Iran and the Shiite-offshoot Alawite sect to which the Assads belong. And Sunni Arabs in
Gulf monarchies funneled aid to the Sunni rebels as they grew increasingly violent.

   Now, the Syrian revolt has fulfilled some of the worst sectarian fears — and
threatened the security not only of the Assad family but also of Iran and Saudi Arabia. The
most-vicious Sunni extremists among the rebels, the Islamic State, have seized a broad expanse of
territory across both states and boast of executing hundreds of Shiites and destroying their
mosques.

   Its rampage has brought it to the doorsteps of both the Iraqi government in
Baghdad, an Iranian ally, and the Saudi Arabian monarchy, which has long feared such extremists as
a threat to its own power at home.

   Across the region, though, the resurgence of Sunni-Shiite sectarian hostilities
has followed a pattern: The weakening of the old states leads anxious citizens to fall back on
sectarian identity, while insecure rulers surround themselves with loyalists from their clans and
denominations, systematically alienating others, often on sectarian lines. In the case of U.S.
allies like Bahrain and Iraq, analysts say, the United States and other Western powers turned a
blind eye to the excesses and sectarianism of rulers they supported.

   Hammering on those internal cracks, the region’s two geopolitical heavyweights,
the Shiite theocracy in Iran and the Sunni monarchy in Saudi Arabia, have sought to protect their
interests and influence by funneling support to clerics, satellite networks, political factions and
armed groups squaring off along sectarian lines.