It is odd listening to our politicians talking portentously about the heavy responsibility they bear in deciding whether Britain should “go to war” in Syria. The sad fact of the matter is that, to all intents and purposes, Britain is already at war in both Syria and Iraq and has been for at least a year.

The current intense debate about whether to step up our involvement by joining the US and France in the aerial bombing of Islamic State terrorists in Syria has been fuelled by the dreadful Paris attacks and fears that something similar may happen here. But as they grapple with their consciences, some of the leading parliamentary participants in this debate exhibit an inexplicable forgetfulness about what they have previously agreed and an inability to recognise what is actually happening, almost daily, in the world beyond Westminster.

Following hard on a series of Isis advances and atrocities, MPs decided on 26 September last year, without much fuss, to support British air strikes in Iraq. All the main party leaderships concurred. The vote in favour was overwhelming – 524-43. Since then, RAF Tornados have conducted hundreds of attacks and sorties.

Facebook Twitter Pinterest French fighter planes. France launched air strikes against Isis targets in Iraq on 23 November from the Charles de Gaulle aircraft carrier, newly deployed in the eastern Mediterranean. Photograph: Anne-Christine Poujoulat/AFP/Getty Images

At the same time, Britain has been contributing airborne intelligence, surveillance and reconnaissance capabilities, refuelling support and command and control assistance to coalition operations in Syria. British drone strikes have killed British-born terrorists in Syria and British pilots have been embedded with coalition forces there. The legal right of collective self-defence, recognised in the UN charter, underpinned the 2014 Iraq intervention and still applies to British armed action in Syria.

Even though some among the Labour leadership do not appear to grasp it, Isis, in its turn, has been waging vengeful war on Britain from its headquarters in Raqqa, Syria, for at least the past 12 months. According to the security services, Isis was behind seven major terrorist plots targeting the UK in this period. It is now busy expanding its “external planning capability” to launch more mass casualty attacks.

Isis has continued to inculcate and recruit young British Muslims, up to 800 of whom have joined it in Syria. It assiduously disseminates a poisonous ideology intended to demean our values and undermine the integrity of our society. And it murders our people. In June, a gunman linked to Isis killed 30 British holidaymakers on a beach in Tunisia. We said then Isis had declared war on Britain and that it was time to wake up. This is even more true today.

Thus the fundamental question that MPs must answer this week, if David Cameron calls a vote, is not whether we should go to war with Isis in Syria – that is already happening – but how we may best ensure that we eventually prevail. And here it is easy to become bogged down in complex and often contradictory geo-strategic, political, historical and military considerations that can be twisted around to support almost any conclusion, depending on your point of view.

It is true, for example, as Mr Cameron says, that “the longer Isis is allowed to grow in Syria, the greater the threat it will pose”. Study of the group’s core beliefs, which centre on an exclusive caliphate governing all Muslims whose desired destination is the “end of times”, meaning the Apocalypse, shows how futile is any hope of a negotiated settlement or compromise. But it is also true, as many argue, that extended RAF bombing cannot, by itself, have a definitive impact and may even make matters worse.

Such decisions are not simple or straightforward. If they were, we would not now be having such a difficult national conversation. Nor is there a single “correct” solution. Even the most passionate advocates of bombing, and their most dedicated opponents, must admit there is room for error in their judgments and a need for flexibility in their approach. And so when Mr Cameron sets out his best case in favour, as he did in the Commons last Thursday, he and his arguments deserve to be heard with attention and respect.

Facebook Twitter Pinterest David Cameron addresses the House of Commons, making his case for air strikes on Syria. Photograph: AP

The prime minister made many persuasive points. He is right to say that Britain cannot and must not depend on others for its defence and that no action comes without risk. It is important, too, that we show solidarity with our long-time ally, France, after the great hurt it has sustained. There is no doubt that Britain faces an extremely serious security threat and that, regretfully, the likelihood of a successful Isis operation on British soil is growing. If we do not suppress Isis now, then when? he asked.

Mr Cameron was correct to stress the twin imperatives of encouraging and assisting the moderate political and armed opposition to the Assad regime in Damascus and of simultaneously trying to advance the Vienna peace process that recently revived moribund negotiations first begun in Geneva in 2012. Mr Cameron rightly highlighted Britain’s humanitarian assistance to the Syrian people, which has indeed been significant, in terms of cash, if not refugee placements, by comparison with most other countries. And recalling mistakes made in Iraq, he placed welcome emphasis on ensuring adequate, collective funding for Syrian postwar reconstruction.

These were key elements in what he claimed was an internationally agreed “comprehensive strategy”. But while not rejecting much of Mr Cameron’s statement, and without questioning his sincerity, it must be said the prime minister’s proposal, taken in the round, is neither comprehensive nor a strategy. On the contrary, it is, to a worrying degree, an exercise in wishful thinking. Mr Cameron has not yet made a convincing case for expanded military action in Syria.

There are three main areas of unresolved, critical concern. First, it remains wholly unclear how additional British bombing, however skilful and accurate, would further the primary objective of dislodging Isis from Raqqa and the rest of its territory in Syria and Iraq, an essential aim if the pernicious concept of a reconstituted, omnipotent caliphate is to be exploded. Only substantial, well-trained ground forces could do this – and western troops are not available. Nobody, not even the French, is prepared to risk them.

Notwithstanding the suspiciously chimerical army of 70,000 rebel fighters conjured by Mr Cameron from the northern deserts, capable local ground forces are mostly lacking. In a state fracturing along religious and ethnic fault lines – with Shia-dominated Iran supporting the dictatorship of Bashar al-Assad, himself a member of the Shia Alawite sect, in a deadly struggle with predominantly Sunni forces and Kurdish separatists, in addition to the murderous Isis – they may be impossible to muster. The Kurds, responsible for Isis’s few significant battlefield reverses to date, chiefly wish to secure their own territory, not fight a liberation war for all Syria. And Syria’s Sunni Arab fighters (as became the case in post-surge, post-Petraeus Iraq in 2007) are divided, unreliable and badly led.

Facebook Twitter Pinterest Syrian Kurdish-led forces. Photograph: Reuters

And even if Isis were routed on the ground, what then? Another vacuum in another ungoverned space, to be filled by whom? Iranian-commanded Shia militia from Baghdad? Assad army forces backed by Russian air support? If Mr Cameron, possibly backed by France, is seriously suggesting (as his memorandum to the foreign affairs select committee implied) that the international community cut a deal with Assad’s regime to tackle Isis and so secure the lands of eastern Syria, he must think again. The Syrian people, not least the Sunni Muslim majority, would never stand for it. Neither would the Gulf states and many other coalition members, whatever Russia and Iran might say.

It is at this point that the spectre of Iraq after the 2003 invasion looms especially large. Saddam Hussein’s regime, though secular in nature, was dominated by and protective of the country’s Sunni minority. After the dictator fell, Iraq’s bigger Shia population eventually seized dominant governmental control. Many Sunnis felt disenfranchised. Their alienation in turn spawned the Sunni rebellion against the American occupiers in 2005 and the rise of al-Qaida in Mesopotamia, the forerunners of Sunni-led Isis. Although they are a majority in Syria, Sunni Muslims were also marginalised by Assad’s Alawite Shia regime. It is they who have stood at the heart of the anti-Assad uprising since it began in 2011.

Thus if a lasting political settlement is to be reached in either country, it is absolutely essential that Sunni interests and aspirations are fully recognised. If Isis and other extremist groups are to be defeated and permanently excluded and inclusive, democratic governance established, the victorious charge must ultimately be led by moderate, indigenous Sunnis, not by vengeful western politicians or well-meaning UN mediators or self-interested Kurds or Tehran-backed Shia militia. Their presumptuous intervention now, as in the past, would only exacerbate existing divisions.

Facebook Twitter Pinterest Civilians walk along a damaged street after what activists said was shelling by forces loyal to Syria’s President Bashar al-Assad in Damascus. Photograph: Bassam Khabieh/Reuters

Practical, credible measures to attain this political imperative are nowhere to be found in Mr Cameron’s strategy. Likewise, the overriding necessity, in Syrian eyes – of deposing Assad – is given no great priority and, indeed, is curiously downplayed. Whatever Mr Cameron may pretend, Tehran’s leadership is not onside – it still argues, absurdly, that Assad should stand for re-election at the end of a transitional period. Yet the prime minister’s plan deliberately bypasses such complexities. By suggesting he has the solutions, and promising what he cannot deliver, Mr Cameron, like Tony Blair in 2003, stretches public trust in the government’s case to breaking point.

Second, Mr Cameron’s comprehensive strategy places far too much faith in a positive outcome in the Vienna peace process, which has yet to produce anything much beyond a deceptive show of international amity. Its overly optimistic 18-month Syrian transition road map is a mere paper trail so far. Assad shows no sign of stepping down, Moscow and Tehran show no sign (as yet) of pushing him, the main Syrian opposition coalition says it will boycott any talks while Assad is still involved, and there is no agreement on which of the many other rebel groups are “moderate” enough to attend the talks. This is treacherous ground from which to launch a military campaign.

Likewise, the “global coalition” energetically promoted by President Hollande since the Paris attacks and backed by Mr Cameron, which in reality boils down to a faint hope of increased military co-operation between Moscow and Washington, fails to inspire confidence. Entirely lacking is a vision for a Syria whole and free. Instead, Britain’s international partners can often resemble vultures, contesting the dismembered parts. The Saudis and Qataris remain at total odds with Iran, the Russians and the Turks are at each other’s throats over border violations, and there is still no broad agreement with Moscow (despite Mr Hollande’s valiant efforts) over who should actually be bombed and where. Indeed, President Vladimir Putin increasingly plays the role of cowboy, not collaborator.

In the meantime, Britain’s closest ally, the US, is running on idle, with an insouciant, strangely detached Barack Obama waxing ever more philosophical, though grimly determined not to be sucked into another Middle Eastern quagmire. And as long as Assad, the original villain of the piece, stays, the refugees, who fear him more than Isis, will just keep on coming. As we report today from the Turkey-Syria border, the very last thing those Syrian civilians who remain want is yet more heavy ordnance descending on their heads.

Facebook Twitter Pinterest Barack Obama: determined not to be sucked in. Photograph: Yin Bogu/Xinhua Press/Corbis

Third and last, there is scant ground for confidence that Mr Cameron’s comprehensive strategy will go any significant way towards addressing the core problem of contagion. By this is meant the apparently inexorable, global spread of jihadi ideology and the terrifying willingness of far too many young Muslims, from London and Birmingham and Bradford, from Lille and Brussels and Copenhagen, and from many other parts of Europe and North America, to reject the liberal values, standards and democratically agreed, albeit imperfect laws of their home countries – and to resort instead to extremist, fundamentalist thinking, arbitrary, divisive and misogynistic norms and the most awful violence to express their vision of a better society.

Isis did not invent the idea of a holy war or crusade. It has no monopoly and no patent, historical or modern, on fanatical, religiously inspired intolerance. Nor did it invent fascism or apocalyptic nihilism, although its beliefs and methods ape both. If Isis is destroyed but not discredited, debunked and exposed for the evil sham it is, other, possibly ever more extreme Islamist groups will likely take its place. As we said at the beginning, Britain is already at war. It is already assisting its French and American allies militarily. But at bottom, it is a war of values and ideas, not of more and more bombs.

This is the fight we must win, however long it takes. Mr Cameron has failed to make the case for expanded military action in Syria. His proposal should not be supported.