FOR two sects united by their belief in one Maker, one Book and one Prophet, the amount of blood spilt in the name of their respective faiths by Shias and Sunnis is truly staggering. This is specially so when one considers the tiny differences that define and divide them.

Since the earliest days of Islam in the 7th century when the schism first tore the young Muslim community apart, the two sects have been warring incessantly. Untold thousands have been killed over the years, and this internecine war continues to devastate communities and nations.

I am not qualified to go into the rights and wrongs of this old conflict. However, as a student of history, I can think of no other single cause of disunity among Muslims as this corrosive, centuries-old struggle. Other religions have gone through periods of sectarian violence: witness the bloody religious wars that Catholics and Protestants fought in Europe.

But while these tensions have mostly died down with the slaking of religious passions among most Christians, Muslims continue to fight over whose version is the true Islam. Indeed, much of Islamic history is written in the bloodshed either over succession, or in sectarian wars.

First, Ottoman rule across large parts of the Arab world held Shia-Sunni violence in check, even though in many provinces, Shias were subjected to discrimination. But as this vast area was controlled from Constantinople, open warfare was rare. Then, in the post-Ottoman, colonial era in the last century, European powers largely prevented Shia-Sunni tensions from breaking into hostility.

In the last half of the 20th century, after the departure of colonial forces, many Muslim countries were ruled by secular dictators who, for all their many faults, kept the lid on these ancient sectarian tensions. From Saddam Hussein of Iraq, to Muammar Qadhafi of Libya, to the Assads of Syria, and to Hosni Mubarak of Egypt, violence between the sects was kept at a minimum.

Nevertheless, the ruling sect did marginalise the other: thus, the majority Shias under the Sunni Saddam fared badly. Now the shoe is on the other foot, and it is the minority Sunnis under the Shia heel. In Syria, the minority Alewites have ruled since the Seventies. In Sunni Saudi Arabia, the Shias are marginalised.

The list goes on, but one thing is clear: both sects harbour deep distrust of each other. Indeed, in a recent Pew Institute survey on attitudes in the Muslim world, only 53 per cent of those surveyed in Pakistan considered Shias to be Muslims. This figure is even lower in several other Muslim countries.

There is similar doubt on the other side, with many Shias casting doubt on Sunni beliefs. So clearly, time has only sharpened this schism, rather than healing old wounds. But while more often than not, these tensions are limited to neighbourhoods and nations, the emergence of a Shia theocracy in Iran has taken these differences to a new level.

Although mediaeval Islam saw states engaging in sectarian warfare, this tendency was later suppressed in modern times, as we have just discussed. However, although the Iran-Iraq war was fought over territorial claims and counter-claims, overtly sectarian symbolism was deployed by both sides.

And when the Americans invaded Iraq in 2003, the ruling Baath Party was ousted, and the minority Sunnis displaced from power. This led to a Shia revival, and a major gain in Iranian influence. Indeed, the US-led campaign was widely viewed in the Middle East as having enhanced Iranian power across the region.With friendly Shia governments in Iraq and Syria, Iran could easily send arms to Hezbollah in Lebanon and Hamas in Gaza. Together with its nuclear aspirations, the Iran of the ayatollahs sent alarm bells ringing in Sunni capitals in the region.

Thus, when the Arab Spring reached Syria a couple of years ago, protestors were supported by Sunni Turkey, Saudi Arabia and Qatar. And seeing an opportunity to cut a hostile Iran down to size, western powers have now thrown their weight behind the anti-Assad forces.

But supporting Sunni fighters is proving tricky, given the penetration of the Syrian opposition by Salafi groups that have flooded into the country. Indeed, they are proving to be the most effective and organised among all those currently trying to overthrow their Alewite rulers. The presence of these extremists has made western powers wary of supplying them with lethal anti-aircraft missiles. The fear is that these weapons could be turned against Israeli and western aircraft.

Both Iran and Hezbollah are doing their best to keep the tottering Assad regime in power. They know that a hostile, Sunni-dominated government in Damascus would make life difficult for both of them. Increasingly, the secular Syrian resistance is being sidelined by extremist forces.

The real danger is that Syria will fragment along sectarian and ethnic lines. This would cause chaos in the region, with the spill-over being specially lethal for Lebanon, a country delicately poised over several religious and sectarian fault-lines. Although the great sectarian divide is now playing out in the geopolitical arena, violence between Shias and Sunnis is becoming increasingly bloody in countries like Pakistan. In the subcontinent, the two communities have lived peacefully side by side for centuries. Even though there were occasional clashes at Ashura, there was little of the organised killings that are taking place with sickening regularity in Pakistan today.