President Barack Obama’s decision to go back into the Iraq from which he extricated the United States almost three years ago is risky but right.

The onslaught by the Islamic State has caught defenceless minorities, such as Christians and the Yazidis, in its path, leaving them with the ultimatum to convert or die.

The self-governing northern Iraqi region of Kurdistan could be over-run. There is a real danger of humanitarian catastrophe combined with a strategic disaster — a new jihadistan at the heart of the Middle East, with a corridor to the Mediterranean. The Islamic State is on the march and has to be stopped. Only the US has the wherewithal to halt its advance.

The US has airdropped food and water to about 40,000 Yazidis stranded on a mountain near Mosul after the Islamic State took their traditional home of Sinjar this week, along with a string of Christian towns.

Mr Obama authorised air strikes to defend them from the slaughter the Islamic State has sulphurously promised them — calling this syncretic sect, which draws from Islam, Christianity and Zoroastrianism, devil-worshippers. US forces bombed the Islamic State artillery batteries on Friday, saying they were threatening Erbil.

He also sanctioned action if the jihadis advance further towards Erbil, the capital of the Kurdish Regional Government (KRG), to protect US diplomats and special forces deployed there after the Islamic State seized Mosul and Tikrit in a lightning offensive in June.

The black flags of the jihadis are less than 50km away from Erbil and the Islamic State has forced the withdrawal of KRG’s peshmerga forces along a long border that now looks indefensible.

The dismemberment of Iraq that began under Saddam Hussein and accelerated after the 2003 Anglo-American invasion has given way to an imploding state, with a savage, would-be totalitarian caliphate dancing on its grave.

STRATEGIC CALCULATIONS INVOLVED

There are three imperatives in this treacherous situation: Humanitarian, strategic and political. They will need to be carefully managed.

The case for humanitarian intervention is unarguable against what Mr Obama has rightly identified as incipient genocide.

As the Islamic State practises sectarian cleansing across the plains of Nineveh — with beheadings and crucifixions of infidels and apostates or pretty much anyone who opposes them — anyone who can do something must. The US has been asked to do so by Iraq and can do so under the United Nations doctrine of the right to protect. The UN Security Council should emphatically endorse this.

Strategically, this will mean forcing the Islamic State back through more air strikes and providing the peshmerga — outgunned and fighting in the plains rather than their Kurdish mountains — with heavy arms.

Neither Turkey nor Iran, traditionally nervous about their restive Kurdish minorities, is likely to object as the jihadi tide comes ever closer to their borders. But Iraqi politics are vital. The US held fire in order to force Iraq’s politicians to ditch Mr Nouri Maliki — the Shia Islamist Prime Minister and former US protege whose sectarian policies alienated the Kurds and drove the Sunni tribes into the arms of the jihadis — and replace him with an inclusive government.

Otherwise, America and its allies would look as though they were taking sides with Iran and its Shia allies against the Sunni minority that the US ejected from power in Iraq and the Sunni majority in Syria, whose revolt has been crushed by the Assad regime and usurped by the Islamic State.

Western Friends of Syria have watched this happen with folded arms.

Minorities in northern Iraq face extinction. Iraq itself will disappear if the jihadis are not stopped.

On Friday, Grand Ayatollah Ali Sistani, Iraq’s paramount Shia cleric, all but ordered Mr Maliki to make way and let Iraq regroup and resist.

Mr Obama surely has a mandate against this savagery. Iraq combines the real danger of humanitarian catastrophe with a strategic disaster. THE FINANCIAL TIMES