Yet until just a few decades ago these differences didn't seem to matter much (not least because Shiites only make up a tenth or so of the world's Muslims, and tend to be dispersed across many countries, often as relatively small minorities). That changed dramatically, however, in 1979, when the Islamic Revolution in Iran suddenly installed a militant Shiite regime in one of the Middle East's most populous countries.
"This fundamentally upset the regional balance of power," says Olivier Roy, a leading scholar on Islam at the European University Institute in Florence, Italy. By profiling itself as the new vanguard in the fight against Israel, says Roy, Iran was in a position to challenge the claims of hitherto dominant countries such as Egypt or Saudi Arabia. The Iranians also began sponsoring their sectarian cousins in places like Lebanon and Iraq. "So the Shiites became politicized," notes Roy. The trend accelerated after the U.S.-led invasion of Iraq in 2003, which toppled Saddam Hussein and finally put representatives of the majority Shiites in power there for the first time (though the result can hardly be described as a triumph for democracy, given the authoritarian drift under current Prime Minister Nouri al-Maliki.)
The other trend is what Roy calls "the Salafization" of Islam. The Salafis -- staunch religious conservatives who have much in common with the puritanical outlook of the Wahhabis in Saudi Arabia -- have been steadily rising in influence around the Middle East over the past decade, a trend more recently reinforced by the Arab Spring. In 1959, Roy points out, a leading Sunni scholar published a fatwa that described Shiism merely as one of the recognized "schools" of Islam. Even the members of the Muslim Brotherhood have generally had relatively few negative things too say about Shiism. "But now we have new generation of salafi preachers who consider the Shiites to be heretics, who say that Shiites are not mainstream Muslims," says Roy. "And this is new."
Though all Salafis aren't necessarily militants, the anti-Shiite sentiment is one that they share with Sunni jihadist movements. No one hates the Shiites more than al Qaeda or the Taliban. And, indeed, the Iraqi branch of al Qaeda duly claimed responsibility for the recent bombings in Baghdad. (Iran, for its part, has seized upon the killings in Quetta to assail Islamabad for its failure to protect Pakistani Shiites.)
Now, it's certainly true that we shouldn't accept all narratives about Shiite-Sunni polarization at face value. In places like Iraq, sectarian distinctions are often blurred by intermarriage. Members of the opposition in Bahrain are fond of stressing that their fight against the monarchy is motivated less by religious sectarianism than by a longing for greater political rights -- an aim they share with many Sunnis in the country. (And yes, there's no question that the Bahraini royal family -- like certain other authoritarian regimes in the region -- has been happy to play up the sectarian card, eagerly ascribing any legitimate dissent to Iranian scheming.)