Like a ticking timebomb left unattended for too long, Yemen’s undeclared civil war has suddenly exploded into a region-wide crisis that will have far-reaching, unpredictable international consequences, not least for Britain and the US.

The conflict, spreading outwards like a poison cloud from the key southern battleground around Aden, pits Saudi Arabia, the leading Sunni Muslim power, plus what remains of Yemen’s government against northern-based Houthi rebels, who are covertly backed by Shia Muslim Iran.

What has until now been an unacknowledged proxy war between Iran and Saudi Arabia, the two great powers of the Middle East, has now burst into an open confrontation that appears to be escalating rapidly as other countries and players are sucked in. The primary Saudi aim is to pacify Yemen, but its wider objective is to send a powerful message to Iran: stop meddling in Arab affairs.

The Houthis

The so-called Houthi rebels, also known as Ansar Allah (the Supporters of God), belong to the Zaidi sect, a relatively obscure branch of Shia Islam. Formed by members of the northern al-Houthi clan, the group was originally known as Believing Youth and began life in the early 1990s as a revivalist theological movement reportedly teaching peaceful co-existence.

The group was radicalised by the 2003 US-led invasion of Iraq. Anti-American demonstrations brought the group into conflict with the government of the then president, Ali Abdullah Saleh. In 2004, it launched a fully-fledged insurgency.

The group has sporadically battled both government forces, which have been backed in recent years by US special forces and drones, and Sunni Muslim extremists belonging to al-Qaida in the Arabian Peninsula, which set up bases in Yemen after being expelled from Afghanistan.

Last September the Houthis unexpectedly seized the capital, Sana’a. The Yemeni president, Abd-Rabbu Mansour Hadi, fled to Aden. The Houthis continued their advance southwards, and on Saturday they took the central city of Taiz. Hadi’s whereabouts now are unknown.

The Saudi-led intervention thus appears designed to prevent the entire country from falling into Houthi hands and to support what Riyadh says is the legitimate Yemeni government against its Iranian-backed foes. The Saudis also fear Yemen becoming a failed state haunted by terrorist groups, like neighbouring Somalia.

The former president Saleh, who was obliged to stand down in 2012 after 34 years in power, and his son, Ahmad, are now backing the Houthis against Hadi, the current president. Part of the problem is that both Saleh and his former deputy, Hadi, are divisive figures unable to bring together the country’s many rival forces.

The Iran-Saudi struggle

Iran is widely believed to have trained Houthi fighters and supplied arms since the insurgency began. But this is flatly denied in Tehran. Iran has nevertheless kept up a constant barrage of criticism of Saudi and western efforts to forge a political settlement in Yemen. It appears to see the country in terms of a region-wide struggle for power and influence between itself and Saudi Arabia, a struggle that in turn reflects the Sunni-Shia schism across the Muslim world.

Its first reaction to Saudi-led air strikes overnight was to condemn them as “US-backed aggression”. The foreign ministry in Tehran described the intervention as a dangerous step with unpredictable consequences. “Iran wants an immediate halt to all military aggressions and air strikes against Yemen and its people … Military action in Yemen, which faces a domestic crisis … will further complicate the situation … and will hinder efforts to resolve the crisis through peaceful ways,” the ministry spokeswoman, Marzieh Afkham, said.

It seems possible that the success of the Houthis’ drive south, and the dramatic Saudi reaction in mobilising an international intervention, has taken Iran by surprise. It is unclear how much control Tehran exercises over the rebels.

The long-running rebellion has been a useful, low-cost way for Iran to keep the Saudis off-balance and under pressure in the regional power battle. Now the puppet may have broken loose from the puppeteers. Iran is facing off against Saudi Arabia on other fronts in Syria, the Gulf and not least in Iraq, where the Shia-led government in Baghdad is widely seen to be under Tehran’s influence.

Iranian-backed militia are also leading the current fightback against Sunni Muslim Islamic State forces north of Baghdad, whom Saudi Wahhabi hardliners and groups are said to have funded.

US involvement

The Saudi move has been strongly backed by the US, Riyadh’s principal ally, which is providing “logistical and intelligence support”. It is inconceivable that the Saudis’ stated plan to launch a ground offensive into Yemen employing 150,000 troops would be under contemplation without prior American agreement and support.

As yet, American forces do not appear to be directly involved. But the fact that the Saudis have given the name “Storm of Resolve” to their air operation in Yemen recalls another big joint operation involving US and Saudi ground forces, Operation Desert Storm, the 1991 war to drive Saddam Hussein’s Iraqi forces out of Kuwait.

The Saudi decision to unveil the international coalition in Washington suggests that the Obama administration, rather than the normally risk-averse regime in Riyadh, may be the driving force behind the intervention.

The US also sees this fight as part of a much bigger, strategic struggle with Iran. But little in the Middle East is straightforward. In another regional theatre of war, the Americans find themselves fighting on the same side as the Iranians, using their air power to support Iranian-backed Shia militia attacking Islamic State forces in the Iraqi city of Tikrit.

Incongruous, too, is the prospect of John Kerry, the US secretary of state, meeting his Iranian counterpart this week in Lausanne to try to seal a nuclear deal with Tehran at the same time as the two countries take drastically opposite sides over Yemen. By dramatising the confrontation with Iran, the Saudis may be sending a not so oblique message to Washington that the nuclear deal, which they oppose, is dangerous and that Tehran is not to be trusted.

If so, they will have Israel’s wholehearted backing. Other western and global powers may be drawn in as the crisis unfolds. The Saudi-led intervention has had an immediate, negative impact on world markets, and drove up the overnight oil price by 6%. Import-dependent China was quick to express its concern, though oil-producing Russia (and Iran) will be pleased to see energy prices rising.

Diplomats suggest that Britain, the former colonial power in Yemen and a close Saudi ally, may also be asked for help, if it has not already been approached. “We support the Saudi Arabian military intervention in Yemen following President Hadi’s request for support by ‘all means and measures to … deter Houthi aggression,’” the Foreign Office said. But it said a political solution must ultimately be sought.



Arab joint action

The newly announced coalition is a notable next stage in the already established trend towards Arab countries cooperating in their own defence. In diplomatic terms it also looks like an attempt to send a powerful collective message to Iran to keep out of Arab world affairs.

The joint action will receive official blessing at an Arab League summit in Cairo this week. The coalition includes five of the Gulf Cooperation Council members – Saudi Arabia, Kuwait, the UAE, Qatar and Bahrain – but not Oman, which declined to take part.

Also on board, according to the Saudis, are Pakistan, Morocco, Jordan, Egypt and, most surprising of all, Sudan. Turkey said it supported the action. It seems clear that Riyadh has been calling in debts, asking for the support of countries it has helped financially and diplomatically in the past.

They include the new regime in Cairo which overthrew the Muslim Brotherhood government, loathed in Riyadh, and Pakistan, to which the Saudis have made big loans over the years. Since the Arab spring sent shockwaves through the region’s unelected regimes, and following the rise of extremist groups such as al-Qaida and Isis, Arab leaders have been increasingly inclined towards joint military action to protect their interests. There have been joint interventions, using air power, in Libya, Iraq and Syria, and Saudi forces intervened in Bahrain in 2011 to support the Sunni monarchy against its rebellious Shia subjects.

The reported involvement of Sudan marks a new departure. The government of Omar al-Bashir is under sanctions from Washington, and Bashir is wanted for genocide and war crimes. But Khartoum recently broke with Iran and threw in its lot with the Saudis, in return for diplomatic cover and help with its possible international rehabilitation. Now the Saudis are demanding payback.