David Cameron’s plan for joining the war in Syria is a worrying document, full of wishful thinking about the political and military situation on the ground. It is a recipe for repeating past failures in Iraq, Afghanistan and Libya, by misjudging the strength of potential enemies and allies alike.

Mr Cameron presents a picture of what is happening in Syria and Iraq that reflects what the Government would like to be happening. If he and those responsible for carrying out British policy truly believe these views, then we are in for some nasty surprises.

It is important to know if Isis is getting stronger or weaker in Iraq under the impact of more than 5,432 air strikes, 360 of them by British aircraft, carried out by the US-led coalition. The RAF has launched 1,600 missions, showing how difficult it is to target a guerrilla force from the air and it will face the same problem in Syria.

Mr Cameron says that with coalition air support, Iraqi forces have halted Isis’s advance and “recovered 30 per cent of Iraqi territory”. In reality, the situation is much worse. Isis captured Ramadi, the capital of Anbar province, in May, routing the Iraqi army despite strong air support from the US. The territory it has lost is peripheral to its core areas in Mosul and along the Euphrates. The strongest anti-Isis forces in Iraq are the Shia militias backed by Iran, which the coalition does not support with air power.

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In Syria, allies on the ground are going to be the armed opposition who are supposedly fighting both Isis and Bashar al-Assad. These forces are dominated by the al-Qaeda affiliate Jabhat al-Nusra and Ahrar al-Sham, a Sunni hard-line group allied to Nusra. The one place where the “moderates” had some strength was in the south where they launched a much-heralded offensive called “Southern Storm” this summer, but were defeated.

Mr Cameron’s explanation of his strategy is peppered with references to “moderates” whom he wisely does not identify because their existence is shadowy at best. It would, indeed, be very convenient if such a powerful group existed, but unfortunately it does not.

Mr Cameron’s Government does not seem to have taken on board that it is intervening in a civil war of great complexity and extreme savagery. There is a supposition that, if Assad were to depart, there could be a transitional Syrian government acceptable to all Syrians. A more likely scenario is that the departure of Assad would lead to a collapse of the state and the triumph of Isis and the self-declared caliphate.