If the United States and its allies want to combat the Islamic State jihadists (IS, formerly known as Isis) successfully, they should arrange a ceasefire between the forces of Syrian President Bashar al-Assad and the non-IS Syrian opposition. Neither the Syrian army nor the “moderate” Syrian rebels are strong enough to stop IS if they are fighting on two fronts at the same time, going by the outcome of recent battles. A truce between the two main enemies of IS in Syria would be just that, and would not be part of a broader political solution to the Syrian crisis which is not feasible at this stage because mutual hatred is too great. A ceasefire may be possible now, when it was not in the past, because all parties and their foreign backers – the US, Saudi Arabia, Russia and Iran – are frightened of the explosive advance of the Islamic State. US Secretary of State John Kerry told the US Security Council on Friday that there is room for everybody “including Iran” in an anti-IS coalition.

President Obama was much criticised for admitting that he had no strategy to cope with IS and, despite his address to the nation on 10 September, he still does not have one. Assuming he is not going to send a large US land army to the region, he lacks a credible and effective local partner in either Syria or Iraq with the necessary military force to take advantage of air strikes, even if they are intensified in Iraq and extended to Syria.

Mr Obama won the assent of the House of Representatives last week to train and equip moderate rebels in Syria who are supposedly going to fight both Assad and IS. This is essentially a PR operation, since IS forces 30 miles from Aleppo are poised to move against the last rebel strongholds, while the Syrian army is close to regaining control of the city itself.

Likewise in Iraq, air strikes can only do so much. The government in Baghdad and the Iraqi army are still Shia-dominated and, however much the Sunni in Iraq dislike IS, they are even more frightened of its opponents. The US will try to split Sunni tribes and neighbourhoods away from the fundamentalists as it did in 2007, but there were then 150,000 US troops in the country and al-Qa’ida in Iraq was much weaker than IS. At the same time, it will find it difficult to advance further because, aside from Baghdad, it has already seized the areas where live the 20 per cent of Iraqis who are Sunni Arab. In Syria at least 60 per cent of the population are Sunni Arabs, meaning that IS’s natural constituency is much bigger.

The case for a ceasefire in Syria is cogently argued by Yezid Sayigh of the Carnegie Endowment for International Peace in Beirut in a paper entitled “To Confront the Islamic State, Seek a Truce in Syria”. He rightly says that “both the regime of Bashar al-Assad and the more moderate armed rebels arrayed against it are stretched thin, bleeding badly and in an increasingly vulnerable position …. Each has self-serving reasons to suspend military operations to confront the looming jihadist threat from the east.”

The Syrian army suffered heavy defeats at the hands of IS in July and August, though these were little reported in the West. Mr Sayigh cites figures of 1,100 government soldiers dead in July alone. It has long been clear that the army was short of combat troops and could only fight one front at a time. Mr Assad appears to have calculated that the rise of IS would be to his political advantage because most of the world would prefer him to the fundamentalists. But he underestimated the military strength of IS since they captured Mosul on 10 June.

No truce is likely to happen unless there is pressure on both sides by their outside backers – notably the US, Russia, Saudi Arabia and Iran. Much would depend on how realistic they are: the US and Saudi Arabia still want the departure of Assad, but this has been very unlikely since the second half of 2012. Demanding this at the Geneva II talks in February effectively killed off any diplomatic framework for negotiations to end the conflict. Critics of multilateral ceasefires argue that this would mean accepting that the Assad government stay in place, but the Syrian government is not departing in any case. The Assad government may believe that it is gradually reasserting its authority over the rest of the country, but these advances are at a snail’s pace and its grip on ground regained is fragile. The Syrian army might not be able to withstand an all-out offensive by IS.

IS is growing stronger while its opponents in Syria are weakening. It is recruiting fast in all parts of its caliphate: Mr Sayigh cites opposition reports that it began training 6,300 recruits in Iraq in July alone. A study by the National Security Adviser’s office in Baghdad showed that in the past, where jihadis took over an area with 100 fighters, they could recruit between 500 and 1,000. IS seems prepared for air strikes, evacuating its fighters and heavy weapons from buildings where they are identifiable. US air power did not win the war in Afghanistan and is even less likely to do so in Iraq or Syria.

A ceasefire in Syria would remove one of IS’s strongest cards, which is the fear of the Sunnis that, bad though IS may be, the alternative of government re-occupation is even worse. For its part, the government may fear no longer being able to face Syrians with a stark choice between Assad and jihadis who chop off heads.

The restoration of a more normal civilian life in Syria would be an immense advance. Some of the 3 million refugees and 6.5 million internally displaced people out of a total population of 22 million would be able to go home. There might be a re-emergence of more moderate individuals and groups marginalised or driven underground since 2011.

At the moment, the political landscape in Syria must look good from the point of view of IS. Its opponents are divided. The US is backing a group of moderates who barely exist and wants to weaken the Assad government. In the past week some of the heaviest fighting in Syria has been IS’s attack on the Kurdish enclave of Kobani, also known as Ayn al-Arab, close to Turkey. It is defended by the fighters of the YPG Kurdish militia who are the Syrian branch of the mainly Turkish Kurd PKK which the US labels as “terrorist”.

US policy has an Alice in Wonderland absurdity about it, everything being the opposite of what it appears to be. The so-called “coalition of the willing” is, in practice, very unwilling to fight IS, while those hitherto excluded, such as Iran, the Syrian government, Hezbollah and the PKK, are the ones actually fighting. A truce between the government and moderate rebels in Syria would enable both to devote their resources to fighting IS, as they need to do quickly if they are to avoid defeat.