My take on this is that the various militant groups, including ISIS and their opponents, are well funded and armed by private donors, outside powers and black market oil sales. This funding and support will not end with bombing ISIS.
Air strikes also won't stop new recruits joining ISIS. Only disrupting the ties of locals to ISIS, protecting locals from anti-Sunni government repression or stopping the nepotism and the unequal distribution of oil revenue to the various different communities in the country will do that. And, not sure how air strikes and supposedly 'friendly' ground troops are going to chase ISIS fighters across international borders e.g. into Turkey, given that Turkey shot down a Russian plane, and that Turkey would prefer ISIS to defeat Kurdish forces. So probably not a lot to prevent ISIS fighters using standard guerrilla tactics of dropping back into Turkey and coming back to attack opposition towns and positions. And not sure what the strategy is to deal with ISIS fighters who hide themselves amongst the civilian population - air strikes won't help there.
And regardless of whether ISIS lose territory in Syria and Iraq, they will, while capable through funds and technology, continue a strategy of international terrorism because that has been shown to work in disrupting foreign political and military support for their opponents. ISIS probably reason that if on-going dead US and UK soldiers reduced US and UK popular support for foreign military intervention in Afghanistan and Iraq (not to mention what the sight of dead US soldiers did for the support of US intervention in Vietnam), dead foreign civilians will be just as effective if foreign troops won't meet them on the ground.
With the advances in technology and social media, every year it gets easier for insurgents to reach or communicate with foreign shores to gain support. This support may come in the form of terrorism, which is why there has been a rise in these types of terrorist tactics against soft targets on foreign shores rather than fighting conventionally against the superior military power of foreign troops, which would only result in military defeat for ISIS. The North Vietnamese army took on ground troops and US air strikes in Vietnam without surrendering - some of the North Vietnamese troops had tattooed "Born in the North to die in the South" on their bodies, so I'm not expecting foreign military intervention to reduce extremism in the ME, through it may disrupt some of the militants' supply lines.
Iran's 1979 revolution against the extremely brutal dictatorship of the US-installed Shah has always been a source of inspiration to Arab insurgents/ militants/ guerrilla movements. Even Western thinkers such as Thomas Jefferson, principal author of the US Declaration of Independence, wrote in a letter " ...what country can preserve it's liberties if their rulers are not warned from time to time that their people preserve the spirit of resistance? Let them take arms. The remedy is to set them right as to facts, pardon & pacify them. What signify a few lives lost in a century or two? The tree of liberty must be refreshed from time to time with the blood of patriots & tyrants. It is it's natural manure."
ISIS extremists have their own narrative of the need for bloodshed of tyrants and the need for martyrs. They seem to have a very simplistic belief that their Caliphate will guarantee social and economic security for the local Sunni population, and they don't think the local population need any freedoms that will interfere with the practice of the ISIS brand of Islam, hence the label of "extremists".
And this won't end quickly because revolutions, wars of liberation and counter-revolutions are usually followed by sectarian reprisals, and illegal executions in the local area as people fight to retain privilege and eliminate the competition e.g. the actions of the KKK after the US civil war to resist what they saw as non-Southern values, the violence in post-war Iraq and in Libya.
Given the protracted nature of any 'solution', Cameron's argument for air strikes seems to be that he has been asked to by the French government, who are his allies (and who presumably help stem illegal immigration through Calais and help disrupt terrorism in Britain by sharing intelligence) and so he doesn't want to upset the French. And also that ISIS expansion of territory and oil fields is of course a threat to British national interests. He also seems to be saying that it's not morally right to let our allies face the inevitable terrorist backlash to their air strikes against ISIS, without the UK facing some of that backlash ourselves, and presumably he hopes that UK security services with help from the intelligence services of his political allies will continue to be effective in disrupting that terrorist backlash.
