OFALL the wars that have ravaged the Middle East since the outbreak of the so-called Arab Spring four years ago, the bitter rivalry between the more fanatical adherents of Sunni and Shia Islam has now emerged as the region’s defining conflict.

The deadly series of suicide bomb attacks in Yemen on Friday, which are reported to have claimed the lives of nearly 150 people, is just the latest brutal manifestation of the Sunni-Shia conflict which has resulted in rival forces inflicting widespread bloodshed throughout the Arab world. Iraq, Syria, Lebanon and Bahrain are among the many Middle Eastern states that have been badly affected by the deepening hostility between Sunni and Shia factions. And at the heart of a conflict which threatens to transform the political landscape of the modern Arab world is the deadly rivalry between Saudi Arabia’s Sunni fundamentalist ruling family and Iran’s equally uncompromising Shia-based Islamic revolution.

The Saudis have been on a collision course with their powerful Shia neighbours ever since it was revealed more than a decade ago that the ayatollahs were working on a clandestine programme to develop nuclear weapons. Acquiring an atom bomb would allow Iran to achieve its long-standing ambition to reclaim its position as the region’s undisputed superpower, thereby enabling it to intensify its efforts to export the principles of the Iranian revolution further afield.

Iran’s nuclear ambitions have not surprisingly been bitterly opposed by Saudi Arabia, the Gulf region’s most powerful Sunni state, with the result that both countries are now engaged in fighting a proxy war for supremacy throughout the Arab world.

Nowhere is this bitter dispute more keenly felt than in Yemen, a nation that holds the unwelcome distinction of being the Arab world’s poorest state. For decades Yemen was regarded by most Arabs as Saudi Arabia’s back garden, such was the influence of the Saudi royal family on Yemen’s internal political and economic affairs.

In particular Riyadh demonstrated its stranglehold over Yemeni politics by supporting the rise to power in 1978 of Ali Abdullah Saleh as the country’s president, and then helping in 1990 to

Nuclear ambitions? Iran shows off its military might

negotiate the second reunification of a country that includes the former British protectorate of Aden.

Under Saleh’s rule, Riyadh generally enjoyed cordial relations with the Yemeni government in Sana’a. But two key developments have dramatically changed this cosy arrangement during the past decade. The emergence of al-Qaeda in the Arabian Peninsula (AQAP), an offshoot of Osama bin Laden’s original terror Sunnibased movement which was founded by a group of Saudi dissidents, helped to provoke ethnic, tribal and social tensions that quickly returned the country to a state of open civil war. These tensions were exacerbated by Iran’s decision to support the Houthi rebels, the Shia minority in the north of the country, a decision that further destabilised the country after President Saleh was forced from office following the Arab uprisings in 2011.

For the past four years the Quds force of Iran’s Revolutionary Guards has been smuggling weapons to the Houthis, as well as providing expert military training, with the result that the Shia Houthi militia finally succeeded in seizing control of the capital Sana’a last year, forcing the Western-backed president, Abed Rabbo Mansour Hadi, to seek refuge in Aden.

Last week it was claimed that Tehran was increasing its support for the Houthis with the delivery of a 185-ton shipment of weapons and other military equipment.

The Iranian-backed takeover of northern Yemen certainly represents a major setback for the Saudis, who have a 1,000-mile porous southern border with Yemen to protect. The establishment of a pro-Iranian, Shia regime in Sana’a has also been met with deep resentment by the country’s militant Sunni population, which in recent months has seen AQAP — once regarded as the region’s most deadly terrorist organisation — being replaced by supporters of the Sunni fundamentalist Islamic State of Iraq and the Levant (Isil) movement, which in the past year has seized control of large swathes of northern Iraq and Syria.

While there have been reports of tensions between Isil and AQAP, there can be little doubt that Sunni extremists were behind this week’s deadly suicide bomb attacks in Yemen, which were deliberately targeted as Shia mosques frequented by Houthi militiamen, who comprised the majority of the victims. There will inevitably be speculation that the Saudis were in some way involved in the atrocities, particularly as the suicide attacks coincided with the Houthis mounting aerial bombing raids against the Aden headquarters of President Hadi.

The group that claimed responsibility for the attacks, the previously unknown Sana’a branch of Isil, justified its action by claiming “infidel Houthis should know that the soldiers of Islamic State will not rest until they eradicate them … and cut off the arm of the Safavid [Iranian] plan in Yemen.” No one in Riyadh is going to argue with that.

The Saudis have certainly proved adept at protecting their interests against Iranian incursions in the past. When Iran tried to provoke Shia dissidents in the tiny Gulf state of Bahrain to overthrow the kingdom’s Sunni monarchy, the Saudi military intervened quickly to crush the protest movement.

Whether the Saudis initiate a similar military operation in Yemen will depend to an extent on the outcome of the talks currently taking place between the US and Iran over the future of its nuclear programme.

President Barack Obama is said to be keen to cut a deal with Iran’s president Hassan Rouhani, who yesterday claimed that the talks were taking positive strides and that “there is nothing that cannot be resolved”.

The talks are being viewed with deep scepticism by the Saudis and other countries in the region, including Israel, which fear that Mr Obama is preparing a deal that would allow Iran to retain the technical capability to develop nuclear weapons, even if Tehran gives commitments not to do so.

If that is the outcome, the Saudis will want to have a nuclear deterrent of their own, with the result that a conflict that is being fought with proxies might one day escalate in an all-out nuclear war between Sunnis and Shias.