They have done it again. The US, Britain and regional allies led by Saudi Arabia have come together to intervene in another country with calamitous results. Instead of achieving their aims, they have produced chaos, ruining the lives of millions of people and creating ideal conditions for salafi-jihadi movements like al-Qaeda and Islamic State.

The latest self-inflicted failure in the “war on terror” is in Yemen, where Saudi Arabia and a coalition of Sunni states intervened on one side in a civil war in March 2015. Their aim was to defeat the Houthis – labelled somewhat inaccurately as Shia and pro-Iranian – who had seized most of the country in alliance with the former President Ali Abdullah Saleh, who retained the loyalty of much of the Yemeni army. Yemeni politics is exceptionally complicated and often violent, but violence has traditionally been followed by compromise between warring parties.

The Saudi intervention, supported in practice by the US and Britain, has made a bad situation far worse. A year-long campaign of air strikes was supposed to re-impose the rule of former president Abd Rabbo Mansour Hadi, whose dysfunctional and unelected government had fled to Saudi Arabia. Relentless bombing had some success and the forces fighting in President Hadi’s name advanced north, but were unable to retake the capital Sanaa. Over the last week there has been a shaky truce.

The real winners in this war are al-Qaeda in the Arabian Peninsula (AQAP) which has taken advantage of the collapse of central government to create its own mini-state. This now stretches for 340 miles – longer than the distance from London to Edinburgh – along the south coast of Yemen. AQAP, which the CIA once described as the most dangerous protagonist of “global jihad” in the world, today has an organised administration with its own tax revenues.

Unnoticed by the outside world, AQAP has been swiftly expanding its own statelet in Yemen in 2015/16, just as Isis did in western Iraq and eastern in Syria in 2013/14. Early last year, President Obama contemptuously described Isis as being like a junior basketball team that would never play in the big leagues. Likewise in Yemen, the American and British governments misjudged the degree to which AQAP would benefit from Operation Decisive Storm, the ill-chosen Saudi name for its military intervention that has proved predictably indecisive.

The Saudi intervention turned a crisis into a catastrophe. Some 6,427 people are known to have been killed in the fighting, but these are only the figures for casualties known to the health authorities. Since the UN says that 14.1 million Yemenis, 54 per cent of the population, have no access to health care, this is likely to be an underestimate. Even before the war, Yemen was the poorest Arab nation and its people are now starving or malnourished. OXFAM estimates that 82 per cent of Yemen’s 21 million population are in need of humanitarian assistance.

The disaster is not only humanitarian, but political, and does not only affect Yemen. As in Iraq, Libya, Syria and Afghanistan, foreign intervention energises and internationalises local difference as factions become the proxies of outside powers.

Yemen has always had Shia and Sunni, but it is only recently that sectarian hatred has begun to get anywhere near the level of Syria and Iraq. Saudi Arabia portrays the Houthis as pawns of Iran, though there is little evidence for this, so Yemen is drawn into the regional confrontation between Saudi Arabia and Iran.

A point seldom given sufficient weight is that AQAP is expanding so fast, not because of its own strength, but because its opponents are so weak. The Saudi and Gulf financed media often refer to pro-President Hadi forces as taking territory, but in reality the government-in-exile remains in Saudi Arabia. It recaptured the port city of Aden last summer, but its few officials who are there dare not leave their heavily-defended compound except by helicopter. Even where Saudi-backed fighters advance, they leave anarchy behind them, conditions in which the arrival of disciplined AQAP forces may be welcomed by local people.

I have been struck, ever since the US and British invasion of Iraq in 2003, by the extent to which their whole strategy depends on wishful thinking about the strength and popularity of their local ally who usually, on the contrary, is feared and hated. I seldom spoke to Afghans who truly supported the Taliban, but I was always impressed by the number who detested the Afghan government. Yet when one UN official stated publicly that the foreign powers fighting the Taliban, supposedly in support of the government, had “no local partner”, he was promptly fired.

There was the same lethal pretence by Western powers in Libya and Syria that the rebels they backed represented the mass of the population and were capable of taking over from existing regimes. In reality, the weakening or destruction of central government created a power vacuum promptly filled by extreme jihadi groups.

The dire consequences of the Saudi intervention and the rise of AQAP has been largely ignored by Western governments and media. Contrary to their grim-faced declarations about combating terrorism, the US and UK have opened the door to an al-Qaeda mini-state.

This will have an impact far beyond the Middle East because what makes the atrocities orchestrated by Isis in Paris and Brussels so difficult to stop is that they are organised and funded by a real administrative apparatus controlling its own territory. If one terrorist cell, local leader or bomb expert is eliminated, they can be replaced.

As has happened repeatedly since 9/11, the US and countries like Britain fail to combat terrorism because they give priority to retaining their alliance with Saudi Arabia and the Gulf monarchies, even when their policies – as in Yemen – wreck a whole country and enable al Qaeda and Isis to use the chaos to establish safe havens.