It’s all about the Saudis. No matter how complex the new Yemeni civil war may appear – nor how powerful the Houthi rebels have become in the capital of Sanaa – it’s the Zaidi sect of Shiism which the Houthis represent that frightens the Sunni Wahabi monarchy of Saudi Arabia, and not without reason.

For more than five years, there has been armed conflict between Saudi forces and the Houthis, who at one point captured a low mountain range inside the Saudi border. The Saudis blame the usual suspects: Iran and the Lebanese Hezbollah. The Houthis blame the usual suspects: the Sunnis of Yemen and their Saudi supporters along – you guessed it – with the United States.

But like every crisis in the Middle East, the Yemen conflict, which has followed almost seamlessly from the civil war that brought Nasser’s Egyptian army into conflict with the Yemeni royal family - which was supported by the Saudis - is a little more nuanced than news dispatches might suggest. Indeed, Yemen’s first independent ruler was a Shia Zaidi – not a Sunni - who extended his territory over much of northern Yemen between the world wars.

The Imam Yahya was head of the Zaidi sect, whose beliefs and worship have almost as much in common with Sunni Islam as they do with Shiism, but he struggled against the Saudis when they seized Asir and Najran from what Yahya called “historic Yemen”.

Oxford scholar Euegen Rogan has described the ruthlessness of Yahya’s successor, his son Ahmed, who imprisoned and executed his rivals, opened diplomatic relations with the Soviet Union and China but then found himself confronted by Nasser’s call for the overthrow of “feudal regimes” in the Middle East.

Ahmad was not averse to condemning Arab socialism in verse (stealing private property was “a crime against Islamic law”). When Ahmed’s son Badr was overthrown in a military coup, Nasser supported the new republic and the Saudis sought to destroy it, supporting the Shia Zaidi rebels.

In pictures: Global refugee crisis Show all 45 1 /45 In pictures: Global refugee crisis In pictures: Global refugee crisis Yemeni refugees Yemeni refugees carry water to their tent at the Mazraq internally displaced people's camp in the northwestern province of Hajja In pictures: Global refugee crisis Yemeni refugees A displaced man from Yemen's Saada province amid UNHCR tents at a camp set up by the United Nations High Commissioner for Refugees in Mazraq in Yemen's Hajja region, 360 kms northwest of Sanaa In pictures: Global refugee crisis Yemeni refugees Yemeni refugees queue to get food aid at the Marzaq internally displaced people's camp in Harad in the northwestern province of Hajjah In pictures: Global refugee crisis Yemeni refugees Displaced Yemenis from al-Jaachan Al-Ansin, a village in the province of Ibb, some 200km South-East of Sanaa, stand next to their tents in a makeshift refugee camp in Sanaa In pictures: Global refugee crisis Yemeni refugees Yemeni refugees walk to a refugee camp in the southern Saudi province of Jizan after crossing the border from Yemen into Saudi Arabia In pictures: Global refugee crisis Syrian refugees Syrian refugees arrive in Turkey at the Cilvegozu crossing gate of Reyhanli, in Hatay. The number of people driven from their homes by conflict and crisis has topped 50 million for the first time since World War II, with Syrians hardest hit, the UN refugee agency (UNCHR) said, in an annual report released on World Refugee Day In pictures: Global refugee crisis Syrian refugees Syrian refugees walking among tents at Karkamis' refugee camp near the town of Gaziantep, south of Turkey In pictures: Global refugee crisis Sudanese refugees South Sudanese refugees waiting for food in the Kule refugee camp near the Pagak Border Entry point in the Gambella Region, Ethiopia In pictures: Global refugee crisis African refugees African refugees live homelessly at a temporary shelter beside a road on World Refugee Day in Sana'a, Yemen. The number of African refugees who have come to Yemen during the past few years has reached 750,000, most of them are Somalis In pictures: Global refugee crisis Iraqi refugees An Iraqi refugee girl from Mosul stands outside her family's tent at Khazir refugee camp outside Irbil, 217 miles (350 kilometers) north of Baghdad, Iraq. The militants' capture of Iraq's cities of Mosul and Tikrit makes their dream of a new Islamic state look more realistic. It already controlled a swath of eastern Syria along the Euphrates River, with a spottier presence extending further west nearly to Aleppo, Syria's largest city. In Raqqa, the biggest city it holds in Syria, it imposes taxes, rebuilds bridges and enforces the law - its strict version of Shariah In pictures: Global refugee crisis Iraqi refugees Refugees queue to register at a temporary camp in northern Iraq Getty Images In pictures: Global refugee crisis Syrian refugees A young Syrian refugee stands near jerry cans used to collect water at Al-Zaatri refugee camp in the Jordanian city of Mafraq, near the border with Syria. The United Nations hopes that political talks between the warring sides in Syria will clinch local ceasefires to allow vital food and medicines to reach millions of civilians In pictures: Global refugee crisis Syrian refugees A child refugee from the northern province of Raqqa in Syria, reacts from the cold weather in a Syrian refugee camp beside the Lebanese border town of Arsal, in eastern Bekaa Valley Reuters In pictures: Global refugee crisis Syrian refugees Boys help their father remove snow in front of their tent in the Azaz refugee camp In pictures: Global refugee crisis Syrian refugees A Syrian refugee family from Aleppo crosses the Bosphorus from Uskudar to the European side of Istanbul In pictures: Global refugee crisis Syrian refugees A child refugee stands next to a home constructed using a billboard in the settlement of Qab Elias in the Bekaa Valley, Lebanon PA In pictures: Global refugee crisis Syrian refugees Refugee baby Rim in the settlement of Qab Elias in the Bekaa Valley, Lebanon PA In pictures: Global refugee crisis African refugees Refugees arriving at a camp near Bossangoa, 190 miles north of Bangui, the capital. Forty-one thousand people fled their homes following mass executions in the area Juan Carlos Tomasi/Médecins Sans Frontières/Doctors Without Borders In pictures: Global refugee crisis Syrian refugees Representatives of Syrian President Bashar al-Assad, a deeply divided opposition, world powers and regional bodies started a long-delayed peace conference aimed at bringing an end to a nearly three-year civil war In pictures: Global refugee crisis Iraqi refugees A women and a girl wash at a tap at a temporary displacement camp set up next to a Kurdish checkpoint in Kalak. Thousands of people have fled Iraq's second city of Mosul after it was overrun by Isis (Islamic State of Iraq and Syria) militants. Many have been temporarily housed at various IDP (internally displaced persons) camps around the region including the area close to Erbil, as they hope to enter the safety of the nearby Kurdish region In pictures: Global refugee crisis Iraqi refugees Families arrive at a Kurdish checkpoint next to a temporary displacement camp in Kalak In pictures: Global refugee crisis Iraqi refugees An Iraqi refugee girl from Mosul stands outside her family's tent at Khazir refugee camp outside Irbil, 217 miles (350 kilometers) north of Baghdad, Iraq. Days after Iraq's second-largest city fell to Isis fighters, some Iraqis are already returning to Mosul, lured back by insurgents offering cheap gas and food, restoring power and water and removing traffic barricades In pictures: Global refugee crisis Iraqi refugees A girl, who fled from the violence in Mosul, carries a case of water at a camp on the outskirts of Arbil in Iraq's Kurdistan region In pictures: Global refugee crisis Iraqi refugees A displaced Iraqi woman washes her family's laundry as the children shower outside their tent at a temporary camp set up to shelter civilians fleeing violence in Iraq's northern Nineveh province in Aski kalak, 40 kms west of the Kurdish autonomous region's capital Arbil In pictures: Global refugee crisis Iraqi refugees Iraqi refugees from Mosul arrive at Khazir refugee camp outside Irbil, 217 miles (350 kilometers) north of Baghdad AP In pictures: Global refugee crisis Sudanese refugees The international Red Cross said that the road from Bor to the nearby Awerial area 'is lined with thousands of people' waiting for boats so they could cross the Nile River and that the gathering of displaced 'is the largest single identified concentration of displaced people in the country so far' In pictures: Global refugee crisis Sudanese refugees People unload the few belongings at Minkammen, that they were able to bring with them to the camps In pictures: Global refugee crisis Sudanese refugees Thousands of exhausted civilians are crowding into the fishing village of Minkammen, a once-tiny riverbank settlement of a few thatch huts 25 kilometres (20 miles) southwest of Bor In pictures: Global refugee crisis Sudanese refugees Many people had spent days hiding out in the bush outside Bor as gunmen battled for control of the town, which has exchanged hands three times in the conflict, and remains in rebel control In pictures: Global refugee crisis Sudanese refugees A young boy pulls his suitcase of belongings as he walks to find a place to rest after getting off a river barge from Bor In pictures: Global refugee crisis Sudanese refugees A displaced family camp under a tree providing partial shade from the midday sun In pictures: Global refugee crisis Sudanese refugees A boy carries a fish, caught from the nearby Nile river, in a cardboard box on his head back to his relatives to eat In pictures: Global refugee crisis Sudanese refugees A mother and her baby, one of the few to have a mosquito net, wake up in the morning after sleeping in the open In pictures: Global refugee crisis Sudanese refugees Surrounded by barbed wire, people wait inside a camp for internally displaced families located inside a United Nations base in Juba In pictures: Global refugee crisis Sudanese refugees Ajay Galuak Akoy (10) carries water in Bor In pictures: Global refugee crisis Sudanese refugees Hungry women eat leaves from the lalob tree in a camp for internally displaced people in Manangui. The tree (Balanites aegyptiaca) is a common "hunger food" in the region In pictures: Global refugee crisis Sudanese refugees Four-month old Haida Majzub was born in the Ajuong Thok refugee camp inside South Sudan. The camp, in northern Unity State, hosts thousands of refugees from the Nuba Mountains, located across the nearby border with Sudan In pictures: Global refugee crisis Sudanese refugees A girl fills a container with muddy water in the Ajuong Thok Refugee Camp In pictures: Global refugee crisis Sudanese refugees A girl carries water from a communal water point inside a camp for internally displaced families located inside a United Nations base in Juba In pictures: Global refugee crisis Sudanese refugees The clashes in South Sudan began when uniformed personnel opened fire at a meeting of the governing party, the Sudan People's Liberation Movement In pictures: Global refugee crisis Myanmar refugees 45 year old Dilbhar looks towards the camera as she stands in the Shamalapur Rohingya refugee settlement in Chittagong district. She escaped to Bangladesh from the Bodchara village in the Mondu district of Myanmar In pictures: Global refugee crisis Myanmar refugees 32 year old Mahada Khatum, 5 year old Hasan Sharif, and 9 year old Umma Kulsum sit outside their home in the Shamalapur Rohingya refugee settlement in Chittagong district. The family escaped violence and discrimination from the Zomgara Baharchara village in the Meherulla district of Myanmar In pictures: Global refugee crisis Myanmar refugees Hamid and his daughter Rajama sit inside their home in the Shamalapur Rohingya refugee settlement in Chittagong district. They fled to Bangladesh from the Dhuachopara village in the Rachidhong district of Myanmar In pictures: Global refugee crisis Afghan refugees Afghan children wait for relief supplies from the Muslim Hands United For The Needy during an aid distribution at a refugee camp on the outskirts of Kabul In pictures: Global refugee crisis Afghan refugees Afghan people carry relief supplies received from the Muslim Hands United For The Needy during an aid distribution at a refugee camp on the outskirts of Kabul

The sad story of Yemen’s partition and eventual (and unhappy) unity, of Sanaa’s 33-year history of dictatorial rule under Ali Abdullah Saleh - himself a Shia Zaidi - and then the inevitable minority claims of disenfranchisement, meant that the Arab awakening – a bloody ‘spring’ indeed in Yemen – would open still-painful wounds.

Saleh’s departure was to produce a new constitution unsatisfactory to the Houthis. The Saudis now feared that the Shia rebels of the north, named after Hussein Badreddin al-Houthi, the Zaidi leader killed in 2004, were supported by Iran and thus – given their own substantial Shia minority – a threat to the stability of the Kingdom itself.

Many were the Saudi claims of Iranian and Lebanese Hezbollah support for the Houthis – and many were the denials of Iran and the Hezbollah – but the growth of al-Qaeda’s Yemeni faction (following, of course, the same Salafist-Wahabi beliefs as Saudi Arabia itself) brought inevitable United States military involvement.

US drone strikes in Yemen, largely unmonitored by the West’s media, were directed against al-Qaeda, supposedly on behalf of the Saudi-supported Yemeni government. But in December 2009, Houthi spokesmen began to catalogue a series of US raids against their own forces, including 29 air raids which killed 120 people in northern Yemeni cities.

The Houthi advance on Sanaa divided the government army’s strength - since it was now battling al-Qaeda (on behalf of the Americans) and the Houthis (on behalf of the Saudis). Al-Qaeda in the Arab Peninsula moved north to fight the Houthis, garnering Sunni support as it did so.