Blood Year: Islamic State and the Failures of the War on Terror. By David Kilcullen. Oxford University Press; 312 pages; $24.95. Hurst; £9.99.

WHEN David Kilcullen, a young Australian army officer who had been seconded to America’s State Department as a counter-terrorism strategist, arrived in Baghdad’s Green Zone in late 2005 he found himself at “Ground Zero for the greatest strategic screw up since Hitler’s invasion of Russia”. Just as it is said that the first world war was the calamity from which sprang all the other calamities of the 20th century, so too was the bungled aftermath of the invasion of Iraq the screw-up from which all other screw-ups followed.

In his new book, Mr Kilcullen gives an unflinching insider’s account of how mistakes and missed opportunities led inexorably to the events of 2014. This is the “blood year” of his title, when Islamic State (IS) began its blitzkrieg through Iraq that culminated in the seizure of Mosul, the country’s second-biggest city, and the approach almost to Baghdad. Eighteen months after Barack Obama assembled his international coalition to “degrade and defeat” IS, the self-styled “caliphate” still holds Mosul and a string of other Iraqi cities, while controlling much of eastern Syria including Raqqa, its “capital” since 2013.

There is plenty of blame to go round. Mr Kilcullen’s contempt for George W. Bush’s defence secretary, Donald Rumsfeld, is searing. If invading Iraq was an irresponsible distraction from the real fight against al-Qaeda, Mr Rumsfeld’s insistence on a “light footprint” strategy meant that the number of troops available to contain the entirely predictable (and predicted) chaos following the removal of Saddam Hussein was “criminally” inadequate.

This was compounded by disbanding the Iraqi army and then dismantling the ruling Baath party, thus rendering most of the country’s qualified middle class unemployable. The conditions for the debacle to come were now complete. It was in the ensuing years of sectarian bloodletting that al-Qaeda in Iraq (AQI), the predecessor to IS, emerged under the psychotic leadership of Abu Musab al-Zarqawi as the most potent force in the insurgency.

Yet Mr Kilcullen is full of praise for Mr Bush’s commitment to the troop “surge” in 2007 led by General David Petraeus. Mr Bush may have made a lousy call over Iraq in the first place, but he understood the moral and reputational imperative to repair some of the damage and was sufficiently flexible to apply new methods when old ones had failed.

The same could not be said for his successor, Mr Obama. As Mr Kilcullen sees it, by the time the new president took office in 2009 Iraq was in a much better place. AQI had been more or less eradicated, the violence had largely abated and, under American guidance, the government of Nouri al-Maliki in Baghdad showed some willingness to share power with Sunnis and Kurds. But Mr Obama, instead of maintaining enough boots on the ground to keep things going in the right direction, headed for the exit at the first opportunity when, in late 2010, he and Mr al-Maliki failed to agree to a deal extending the legal status of American forces in Iraq.

Within hours of the last American soldiers departing, Mr al-Maliki returned to his Shia sectarian ways, betraying promises made to the Sunnis and hollowing out the army (equipped and trained at a cost of $26 billion to American taxpayers), creating a corrupt praetorian guard. With nowhere to turn, the Sunni minority was vulnerable to co-option by a resurgent AQI, which by 2013 was responsible for levels of violence not seen since the surge. With a strategic hinterland across the notional border with Syria and faced with an Iraqi army unwilling to fight for mainly Sunni cities, AQI morphed into ISIS (Islamic State of Iraq and al-Sham) and then into IS.

It was given a helping hand by a feckless Mr Obama, already guilty of “passivity in the face of catastrophe” according to Mr Kilcullen. Had Mr Obama punished Bashar al-Assad for crossing the “red line” by using chemical weapons against his own people, as Mr Obama had promised, the author believes that the regime might have fallen and IS would not have taken hold in Syria.

Mr Bush learned from his mistakes. But in Mr Obama the author sees an ideological stubbornness that prevents him from doing so and that in some ways echoes the theories of Mr Rumsfeld. Mr Obama relies on technology and modest air campaigns to constrain the West’s enemies while refusing to commit the forces on the ground needed to defeat them (his partial U-turn on Afghanistan is typically grudging and insufficient). The longer America leaves IS to embed itself in the cities it has captured the greater the harm it does and the more lethal the force that will eventually be required to remove it, Mr Kilcullen argues.

Unfortunately, this wise and important book ends on an off-note. Mr Kilcullen suggests that perhaps some sort of bargain could be struck with an economically distressed Russia that would lead to a co-operative effort to find a political solution to end the terrible civil war in Syria that IS has fed from. Given Mr Obama’s acquiescence in the ruthless bombing of Syrian rebels ordered by Vladimir Putin just as this book was sent to the publishers, it is not a far-fetched notion. But it would be a deal struck on Russian and Iranian terms aimed at preserving a monstrous regime only too happy to drive yet more desperate Sunni Arabs into the deadly embrace of IS.