As early as November, parliament could be asked to authorise bombing in Syria. It will be a re-run of the 2013 debate with one important difference: this time we will be bombing the other side.

The UK’s national security body was said last week to have recoiled from a proposal to impose a no-fly zone against President Assad’s air force, using ships and cruise missiles. The government is reportedly “nervous” of any military action that might provoke an Iraq-style protest movement. Instead, we will be asked to authorise strikes against Isis, already the target of RAF bombing missions in Iraq. They are right to be nervous, because so is the British public. There is revulsion at the outcome of the Iraq and Afghanistan interventions; despair over the west’s inability to rebuild democracies in the dictatorships it invades.

For a major and historic military power, this is a situation close to paralysis. Its roots lie in the breakdown of the two alliance systems of which Britain is part. China and Russia prevent the UN security council from endorsing lawful military intervention to stop the massacre. And the US has lost its appetite for full-scale military intervention. At Westminster, there is the added complication that the opposition does not yet know its position.

But a decision is coming. Britain, as a permanent member of the UN security council, has not only the right but the duty to uphold international law, by force if necessary.

If you’re a pacifist, then it’s justifiable on moral grounds to say nothing can be done. If you are not, then the most valuable thing you can apply to the Syrian crisis is logic. As a public service journalist, I don’t get a voice on whether to bomb or not. But we all have a right to ask questions.

The first challenge is to understand what is happening. A democratic uprising became a civil war, got hijacked by Islamists, was abandoned by the west, and became a bargaining piece in a bigger standoff with Putin’s Russia. Yes, the US was trying to destabilise Syria before 2007, as the WikiLeaks cables show, but it was as unenthusiastic about the Syrian spring of 2011 as it was about the Arab spring in general. The Syrian uprising failed because Putin re-armed a military that was teetering, and because Iran’s proxy militia in Lebanon, Hezbollah, intervened in a crucial battle that helped defeat the moderate opposition. And because – again at crucial moments – Qatar and Saudia Arabia withheld aid from secular opposition forces in order to strengthen their own proxies. Though the US-backed Free Syrian Army is fighting Assad, major recent gains were made by the so-called Army of Conquest, backed by Turkey, Saudi Arabia and Qatar. In July, the latter actually attacked US-trained rebels inserted into their territory.

So, it’s a four-way civil war. In such a war any military action from outside will provoke a reaction, and a change in the balance of forces. The question those proposing to bomb Isis alone have to answer is: how will that help end the conflict in Syria? If Isis disappears from the map, who does the British government think will take its place? Three-quarters of civilian deaths have been attributed to Assad, not Isis. So is the attack on Isis to be used as a bargaining chip with Assad? Or as an implicit threat to him?

The next question is: what is the desired end result. From the US state department to Jeremy Corbyn’s office in Portcullis House, there is agreement that it will involve a diplomatic settlement including Russia and any western nations who have committed forces. It may lead to partition, or de-facto zones of control. And it must involve the destruction of Isis.

The US has become used to using Saudi Arabia, Qatar and Turkey as proxies in the region, as Russia has with Iran and Hezbollah. But the US nuclear deal with Iran has angered these regional players and strengthened their determination to have their own game plan for the Syrian opposition.

Before it commits to any conflict in Syria, parliament needs to ask what the involvement of these powers will be. For this is a war as dirty as the dirtiest moments of Iraq. The increasingly unhinged regime in Turkey has already used the cover of fighting Isis to bomb Kurdish positions in Iraq and is now at war with the Kurdish PKK inside Turkey itself. The Saudis and Kuwaitis have fuelled sectarianism throughout. Specifically, what promises will be made to Syria’s Kurds as to their position in a postwar carve-up?

The next question is legality. Since Isis has recruited British Muslims and targeted them against Britain, the legal case for proportionate military action will not be hard to make. But an attack on Syria would have to be justified under international humanitarian law. That means proving there is an overwhelming humanitarian need; that there is no alternative; and that the action is proportionate. This, I understand, is the argument that has been put to both frontbenches: that hitting Assad is the single most constructive thing you could do to end civilian deaths and stem the flow of refugees.

The final problem is geopolitical effectiveness. Given Russia has moved troops into Syria this month, and may be preparing to station fighter jets there, why does the British government think bombing Isis, or bombing Assad’s airbases at the same time, would bring Russia closer to a strategic deal? Proponents of intervention say it will, and that Putin will broker peace in Syria if he sees Assad’s freedom of action once again curtailed.

Right now, the public mood in Britain is split between “something must be done” and “nothing can be done”. We employ diplomats and national security advisers to think beyond this: but we have a right to detail, clarity and transparency.

Isis rules half of Iraq because the government the west installed there failed. It rules large parts of Syria because that state fell apart, and the west balked at the chance to fight Assad in 2013. In both Syria and Iraq, then, Isis took advantage of the west’s failure of strategy. Logic therefore dictates that what can beat Isis is not primarily bombs or drones but an effective regional strategy. That’s what I want to hear from the green benches on the day defence secretary Michael Fallon asks for permission to shoot.

• Paul Mason is economics editor of Channel 4 News. @paulmasonnews