Britain is set to join the air campaign against Isis in Iraq, but, going by David Cameron’s speech to the UN General Assembly, the Government has no more idea of what it is getting into in this war than Tony Blair did in 2003.

Mr Cameron says that there should be “no rushing to join a conflict without a clear plan”, but he should keep in mind the warning of the American boxer Mike Tyson that “everyone has a plan until they get punched in the mouth”.

The Prime Minister says that lessons have been learned from British military interventions in Iraq and Afghanistan but it is telling that he did not mention intervention in Libya for which he himself was responsible.

In fact, there is a much closer parallel between Britain joining an air war in Libya in 2011 than Mr Blair’s earlier misadventures which Mr Cameron was happy to highlight.

In Libya, what was sold to the public as a humanitarian bid by Nato forces to protect the people of Benghazi from Muammar Gaddafi, rapidly escalated into a successful effort to overthrow the Libyan leader. The result three years on is that Libya is in permanent chaos with predatory militias reducing their country to ruins as they fight each other for power.

Whatever the original intentions of Britain and the US, their armed intervention in Afghanistan in 2001, Iraq in 2003 and Libya in 2011 has been to produce devastating conflicts that have not ended.

It has become common over the years to describe Iraq as a quagmire for foreign powers and it is no less so today than when President Bush and Mr Blair launched their invasion 11 years ago.

Mr Cameron draws comfort from the fact that the UN Security Council has received “a clear request from the Iraqi government to support it in its military action” against Isis. But this is a government who lost five divisions, a third of its army of 350,000 soldiers, when attacked by 1,300 Isis fighters in Mosul in June. Its three most senior generals jumped into a helicopter and fled to the Iraqi capital Arbil, abandoning their men. It was one of the most disgraceful routs in history.

Mr Cameron blames all this on the mis-government of former Prime Minister Nouri al-Maliki, whose sectarian and kleptocratic rule has just ended. But it is doubtful if much has changed since Mr Maliki was replaced by the more personable Haider al-Abadi, whose government is still dominated by Shia religious parties. Mr Cameron’s stated belief that he is supporting the creation of a government that is inclusive of Sunni, Shia, Kurds and Christians is a pipe dream.

It is important to stress that there is little sign that US air strikes in Iraq, which Britain is planning to supplement, will be able to turn the tide against Isis. There have been 194 US air strikes in Iraq since 8 August but the militants are still advancing six weeks after the first bombs and missiles exploded.

In a little reported battle at Saqlawiya, 40 miles west of Baghdad, last Sunday, Isis fighters besieged and overran an Iraqi army base and then ambushed the retreating soldiers. An officer who escaped was quoted as saying that “of an estimated 1,000 soldiers in Saqlawiya, only about 200 managed to flee”.

Surviving Iraqi soldiers blame their military leaders for failing to supply them with ammunition, food and water while Isis claims to have destroyed or captured five tanks and 41 Humvees. The message here is that if the US, Britain and their allies intend to prop up a weak Iraqi government and army, it is misleading to pretend that this can be done without a much more significant level of intervention.

In 2003, Mr Bush and Mr Blair claimed to be fighting only Saddam Hussein and his regime and were astonished to find themselves fighting the whole Sunni community in Iraq. This could very easily happen again in both Iraq and Syria.

Many Sunni in Mosul and Raqqa, Isis’s Syrian capital, do not like Isis. They are alienated by its violence and primaeval social norms such as treating women as chattels. But they are even more frightened of resurgent Iraqi or Syrian armies accompanied by murderous pro-government militias subduing their areas with the assistance of allied air strikes. The Sunni will have no option but to fight or flee.

The US is hoping that it can split the Sunni community away from Isis in a repeat of what happened in 2007 when many Sunni tribes and neighbourhoods took up arms against al-Qaeda in Iraq (AQI). But this is less likely to happen this time round because Isis is stronger than its predecessor and takes precautions against a stab in the back. Mr Cameron cited the example of the al-Sheitaat tribe in Deir Ezzor in eastern Syria, who rose up against Isis only for their rebellion to be crushed and 700 of their tribesmen to be executed.

Mr Cameron produced a laundry list of four measures that will make the present intervention in Iraq different from past failures. They are a ragbag of suggestions, high on moral tone but short on specificity and give the impression that Tony Blair may have been looking over the shoulder of Mr Cameron’s speech writer.

For instance, we should defeat “the ideology of extremism that is the root cause of terrorism”, but there is nothing concrete about the origins of this narrow and bigoted ideology which condemns Shia as heretics and apostates, treats women as second-class citizens and maligns Christians and Jews.

In fact, the belief system of Isis is little different from Wahhabism, the variant of Islam prevalent in Saudi Arabia. Supported by Saudi wealth, Wahhabism has gained an ever-increasing influence over mainstream Sunni Islam in the last 50 years. Politicians like Mr Cameron are much happier condemning school governors in Birmingham for religious extremism than they are complaining to the Saudi ambassador in London about the virulent sectarianism of Saudi school books.

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The US and British alliance with Saudi Arabia, United Arab Emirates, Qatar, Bahrain and Jordan – all Sunni monarchies – creates other problems. It is hypocritical for Mr Cameron to pretend that US and UK intervention are in support of democratic, accountable and inclusive governments when he is in a coalition with the last theocratic absolute monarchies on earth.

But the most short-sighted and self-defeating part of Mr Cameron’s justification for British intervention is to do with the war in Syria. He still claims he wants to change the government of Syria, a policy in which there is “a political transition and an end to Assad’s brutality”. He adds the shop-worn observation that “our enemies’ enemy is not our friend. It is another enemy.”

Since Mr Assad controls almost all the larger Syrian cities, he is not going to leave power. What Cameron is in practice proposing is a recipe for a continuing war and it is this that will make it impossible to defeat the jihadi militants, for Isis is the child of war.

Its leaders have been fighting for much of their lives and are good at it. They and their followers interact with the rest of the world through violence. And so long as the wars in Syria and Iraq continue, then many in their Sunni Arab communities will fear the enemies of Isis even more than Isis.

What the plans of President Obama and Mr Cameron lack is a diplomatic plan to bring the war between the non-Isis parties in Syria to an end. The two sides fear and hate each other too much for any political solution, but it may be possible for the foreign backers of the two sides to pressure them into agreeing a ceasefire. Neither is in a position to win against each other, but both are threatened by Isis, which inflicted stinging defeats on both Assad and anti-Assad forces in the summer.

Britain should press for such a truce even if it is only engaged militarily in Iraq, because it is the outcome of the war in Syria that will determine what happens in Iraq. It was the Syrian war beginning in 2011 that reignited Iraq’s civil war and not the misdeeds of Mr Maliki.

If Isis is to be combated effectively, then the US, Britain and their allies need to establish a closer relationship with those who are actually fighting Isis, which currently include the Syrian Army, the Syrian Kurds, Hezbollah of Lebanon, Iranian-backed militias and Iran itself. The necessity for this is being made tragically clear in the Syria Kurdish enclave of Kobane on the Syrian-Turkish border, where Isis fighters have already driven 200,000 Kurds into Turkey.

If Mr Obama and Mr Cameron genuinely intend to rely on plans to combat Isis that they have just outlined, then they are, as Mike Tyson would have predicted, setting themselves and their countries up for a punch in the mouth.