A letter printed at the bottom of this article was emailed by a friend soon after her neighbourhood in Mosul was hit by Iraqi airforce bombers. This was some hours before President Barack Obama explained his plan to weaken and ultimately destroy Isis, which calls itself Islamic State, by a series of measures including air attacks. The letter illustrates graphically one of the most important reasons why American air power may be less effective than many imagine.

The reasons for this are political as well as military. The five or six million Sunni Arabs who live in areas controlled by Isis in Iraq and Syria may not be happy with the brutality, bigotry and violence of their new rulers. But they are even more frightened of the prospect of the soldiers and militiamen of the Baghdad or Damascus governments recapturing and wreaking vengeance in Sunni cities, town and villages. The Sunni communities in both countries have little choice but to stick with Isis as their defenders.

For all its bellicose rhetoric, Mr Obama’s plan is more of a strategy to contain Isis rather than eradicate it – and he may find that even this is difficult to do. His problem is that the US does not have reliable local partners in either Iraq or Syria.

The new Iraqi government under Prime Minister Haider al-Abadi remains sectarian, with even more members of the ruling Shia Dawa party than before. The Kurds were press-ganged by the US into joining it though none of their outstanding demands has been satisfied.

In Syria, the US is to bolster the “moderate” Syrian rebels who are to be trained in Saudi Arabia. The Syrian military opposition on the ground is dominated by jihadis, of which Isis, with control of 35 per cent of the country, is the most powerful.

The US air power should be enough to prevent Isis capturing the Kurdish capital Irbil or launching a successful assault on Baghdad. It might also be employed to save Aleppo or Hama from Isis. But without American forward air observers embedded in Iraqi units, as happened in Afghanistan in 2001 and Iraqi Kurdistan in 2003, the Iraqi army is unlikely to make real progress on the ground. Given that the Sunni community is likely to fight the Shia-dominated army to the last man or flee in front of it, this may be no bad thing.

For all Mr Obama’s caution, the US is being dragged into new conflicts in Iraq and Syria. By beheading two American journalists in retaliation for US air strikes Isis has shown that it will retaliate against any US or British attack. Concern is expressed about the possibility of Isis bombers blowing themselves up “in the streets of London” but they could more easily target the 2.5 million British tourists who visit Turkey every year.

The Obama plan is inadequate to “degrade and ultimately destroy” the militants, though it may well be the best that can be done in the circumstances. Events on the ground are likely to drive the extent of US involvement. What would happen, for instance, if Isis made a lunge for Aleppo, something that would be easy to do since Isis forces are only 30 miles away from the city. Would those in Washington who say there must be no co-operation with Mr Assad not use American air power in support of his forces?

There is a bizarre section in Mr Obama’s speech in which he says “we must strengthen the [Syrian] opposition as the best counterweight to extremists like Isil [Isis]”. The only way that this could be done would be to raise a mercenary army and pretend it is the Free Syrian Army reborn or, something that Saudi Arabia and Qatar have done in the past, pretend that jihadi groups whose ideology is the same as that of Isis nevertheless belong to the moderate camp.

The missing element in the Obama plan is the creation of the framework for new peace negotiations between Mr Assad’s government and the moderate opposition such as it is. The Geneva 11 talks got nowhere because Washington insisted that the only topic of negotiations should be the departure of Mr Assad. Since he controlled most of Syria this was not going to happen, so in practice US and British policy was a recipe for an endless war.

So long as the civil war in Syria goes on Isis cannot be beaten: Syrian Sunni in areas under its rule will prefer it to the alternative which is the return of a vengeful government. In Iraq the political and military reach of Isis is limited by the fact that the Sunni Arabs are only a fifth of the population, but in Syria they are three-fifths. Their natural constituency is much greater than in Iraq.

It is likely that in Syria the US will covertly collaborate with the Assad government using intelligence services and third parties. Likewise in Iraq, the US and Iran are evidently pursuing common aims in propping up the Baghdad government. As the letter writer from Mosul points out the Shia militiamen who broke the siege of the Shia Turkoman town Amerli (and ransacked Sunni villages nearby) were aided in their advance by US air strikes. For all his caution, Mr Obama is being sucked into a sectarian civil war of terrible savagery.

Letter from Mosul: Why Isis is seen as the lesser of two evils The bombardment was carried out by the government. The air strikes focused on wholly civilian neighbourhoods. Maybe they wanted to target two Isis bases. But neither round of bombardment found its target. One target is a house connected to a church where Isis men live. It is next to the neighbourhood generator and about 200-300 metres from our home. The bombing hurt civilians only and demolished the generator. Now we don’t have any electricity since yesterday night. Now I am writing from a device in my sister’s house, which is empty. ORDER IT NOW The government bombardment did not hit any of the Isis men. Now I have just heard from a relative who visited us to check on us after that terrible night. He says that because of this bombardment, youngsters are joining Isis in tens if not in hundreds because this increases hatred towards the government, which doesn’t care about us as Sunnis being killed and targeted. Government forces went to Amerli, a Shia village surrounded by tens of Sunni villages, though Amerli was never taken by Isis. The government militias attacked the surrounding Sunni villages, killing hundreds, with help from the American air strikes.

‘The Jihadis Return: Isis and the New Sunni Uprising’ by Patrick Cockburn, published by OR Books, is available at orbooks.com