In categorising the world's major civilisations, Samuel Huntington divided Christian Europe into Western Civilisation and the Orthodox World, but took the every nation from Afghanistan-Pakistan to Tunisia-Morocco to be part of a single entity called the Muslim World. Yet, the clash between Shias and Sunnis is the longest continuously running conflict in the world. It simmers here, boils over there, but there have been no successful moves towards ecumenism in the region, and The Arab Spring has now exposed, on a geopolitical scale, this fault line running through the Middle East. Those living in West Asia have recognised the amplified threat, but the rest of the world has failed to do so, partly because we tend to classify the region exactly as Samuel Huntington did.
The two countries at the fore of crisis are Bahrain and Syria. Bahrain is a Shia majority country ruled by Sunnis, and Syria a Sunni majority country ruled primarily by Shias (Alawis to be exact, members of a heterodox sect who have drawn closer to conventional Shia beliefs in recent decades).
Let's stick with Syria, because it is turning dangerously unstable. Syria's ruling political group is the Ba'ath Party, which espouses a secular, pan-Arabist ideology. The other nation to have been ruled by a faction of the Ba'ath Party was Iraq. Iraq was the mirror image of Syria: a Shia majority nation controlled by a Sunni Arab dictatorship. Since they espoused the same beliefs, it is hardly surprising that Saddam Hussein and Hafez al Assad hated each others' guts.
Seen from today's foreshortened perspective, pan-Arabism seems to have been a convenient way to paper over the Shia-Sunni dispute. When Saddam Hussein successfully rallied Iraqi Shias to his cause in the war against Iran, it suggested that ethnic identity was stronger than denominational identity in the Middle East. Pan-Arabism, though, ended up being a flash in the historical pan, while the Shia-Sunni divide is as strong as ever after 1300 years, and extends well beyond the Middle East into Afghanistan and Pakistan, as recent bombings have tragically demonstrated.
The vote against Syria by the Arab League showed that political alignments now mirror sectarian ones. Iraq and Lebanon, the only two Arab nations with substantial Shia populations as well as some form of electoral democracy, refused to join 18 Sunni-ruled Arab states in condemning the violence unleashed by Bashar al Assad. Should Syria collapse into anarchy, there's a good chance it will lead to a pogrom against Alawis, and that might draw Iran, and perhaps even Iraq and Lebanon, into intervening. In such a case, Israel and Saudi Arabia would not just stand by, and if those two nations were to get involved in the conflict, the entire world would be affected.