The US plan to weaken and ultimately destroy Isis has several political and military weaknesses undermining its long-term success. Air campaigns not supported by ground forces can damage the other side but they do not win wars on their own. Isis has already faced bombardment by US planes in Iraq since 8 August, but it is still fighting the Iraqi army around Baghdad.

Some of the weaknesses of the air war are already apparent since Isis had evacuated its leaders, fighters and heavy equipment from buildings that were targeted. Its fighters avoid large gatherings and mix with the civilian population. The shock effect of being bombed will be the less because the Syrian air force has long been bombing rebel-held cities and towns.

Isis expertise is in guerrilla warfare and it is only recently that it has used columns of vehicles packed with gunmen and heavy infantry weapons. Air superiority over the fruit groves of Diyala province or the palm trees of northern Hilla is difficult to use effectively. Of course, in Syria and Iraq there are ground troops capable of taking advantage of the air strikes, but they mostly belong to armies and militias with whom the US is not meant to be co-operating.

In Syria, the most powerful armed group opposing Isis is the Syrian army, followed by Hezbollah and the Syrian Kurds. In Iraq, three months after the fall of Mosul, the Iraqi army still does not seem able to hold its own against Isis as recent fighting around Fallujah demonstrated. Advances that have taken place have generally been by Shia militiamen under the direction of Iranian Revolutionary Guards officers and Kurdish Peshmerga, though these did not distinguish themselves when Isis attacked Sinjar in August.

In Iraq, the US task is a little easier because at least it is allied to the elected government in Baghdad. But in Syria Isis is somehow to be attacked without aiding the Syrian government or groups that the US and Europeans have previously demonised, such as Hezbollah, from Lebanon, which has acted as shock troops for the Syrian government. Of course this detachment from military realities is not possible, but what is military common sense may be politically embarrassing.

The main Isis offensive at the moment is directed at the Syrian Kurdish enclave centred on the town of Kobane where half-a-million people had taken refuge. Some 200,000 Kurds have now fled across the border into Syria while their militia, the YPG, is fighting it out with Isis.

Air strikes hit eight villages captured by Isis early today, but here the Americans face a political problem. The YPG is the Syrian branch of the Turkish Kurd PKK whom the US labels as “terrorists” and does not want to be associated with. But what does the US do if Isis responds to the air campaign against it by strengthening its forces trying to capture Kobane. It may want to send a message that it remains an enemy to be feared whatever the US does.

ORDER IT NOW

Air power can be made more effective at a tactical level by having forward air observers on the ground calling in air strikes. This worked in Afghanistan in 2001 and in northern Iraq in 2003. But use of these today in Iraq and Syria means the US getting further involved in somebody else’s civil war.

There are other complications. Turkey now says that it is joining the anti-Isis coalition, but it does not want to strengthen the Syrian Kurds and the YPG or help to keep the Assad government in power. It would be interesting to know what the US has said to Turkish President Recep Tayyip Erdogan about their future plans for Syria.

Isis is strong because of its military expertise and religious fanaticism but there are more general reasons why it will be difficult to defeat. Many Sunnis in the areas controlled by Isis do not like it, but they are terrified of the return of the Syrian or Iraqi armies and accompanying sectarian militias.

Isis has been more successful than it ought to have been because of two vacuums which it has been able to fill. One is the vacuum left by the dysfunctional and corrupt Iraqi army and state. The other is the political vacuum created by the absence of leadership in the Sunni communities in Iraq and Syria which is capable of offering an alternative to Isis.