The discredited justifications that preceded the invasion of Iraq still dominate British and American perception of military intervention in Syria. In a similar way in the 1930s, popular revulsion at the lies and exaggerations of First World War propaganda meant that the first accounts of Nazi atrocities were treated with scepticism.

Unsurprisingly, people who feel they were swindled into war 10 years ago by bloodcurdling accounts of Saddam Hussein's non-existent weapons of mass destruction are dubious about their government's claim that President Bashar al-Assad's army used poison gas on a mass scale on 21 August. All the questions that should have been asked in 2003 about Iraq are being asked about Syria: what is the evidence for chemical weapons? How partial are the sources of information? Why should Assad do something so much against his own interests? Would a limited air assault on Syrian military bases deter him from using chemical weapons again, supposing he used them this time, or would it be the first step towards ever-deeper British and American involvement in the war?

All these are reasonable questions and many of them have reasonable answers. Unlike Iraq, it is known that the Syrian army has large supplies of chemical weapons such as sarin and that a mass attack took place. A hundred videos show the dead and dying. Doctors diagnosed the symptoms of gas poisoning. It is highly unlikely that the opposition had enough chemical weapons to simulate a government attack in order to provoke foreign intervention.

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Of course, the use of poison gas was always likely to provoke the United States into action, something Damascus has been desperate to avoid for two years. But this does not mean they did not do it. Stupidity and miscalculation have shaped many wars. Recall that General Reginald Dyer believed he could quell Indian nationalists and strengthen British rule in India by ordering his soldiers to open fire at a demonstration in Amritsar in 1919, killing 379 people (other figures suggest 1,000 died). Whose bright idea was it to police a protest march in Derry with British paratroopers in 1972 on what became known as Bloody Sunday?

What is curious about the past week is the extent to which so many, especially the media and the British Government, misjudged the continuing rawness of the wounds inflicted by the Iraq war. I was in Baghdad for much of the conflict but I was always struck on returning to Britain by the lasting sense of outrage over the decision to go to war expressed even by the most conservative and non-political. As with the Munich Agreement in 1938, it has entered a deep layer of British historic memory, perhaps because people feel they were not only misled but lied to by their own government.

The parliamentary vote and opinion polls show that British governments have exhausted whatever capital of public trust they possessed when it comes to military ventures in the Middle East. Intelligence reports confirming that Assad used chemical weapons simply jog memories of past deceptions such as the "dodgy dossier" of 2003. Credibility lost then has never been regained. The government is like the little girl Matilda in Hilaire Belloc's poem of that name who, having previously called the fire brigade falsely claiming her house was ablaze, burns to death when it does indeed catch fire:

Every time she shouted 'fire!'

They only answered 'little liar!'

Given the way the deceptions and failures of the Iraq war still resonate, no wonder David Cameron denies that military intervention in Syria today has anything in common with what happened in 2003. But the two countries are alike in their political make-up, with deep sectarian and ethnic divisions giving political convulsions an extra edge of fear and hate. Both were or are ruled by a single extended family or clan monopolising authority in a police state in which power is exercised through the intelligence and security services. They are tough nuts to crack: "Assad is as coup-proof as Saddam ever was," says an Iraqi leader, who has spent much of his life trying to get rid of the latter.

In one crucial respect Assad is in a stronger position than Slobodan Milosovic in Serbia, Saddam Hussein in Iraq or Muammar Gaddafi in Libya. These three leaders were internationally isolated, while Assad has powerful and committed foreign allies. Russia is standing firmly by Assad as it reasserts its status as a great power after 20 years of retreats and humiliations that culminated in the Libyan war of 2011. It feels it was double-crossed then into agreeing to humanitarian military intervention by Nato which swiftly became a campaign to overthrow Gaddafi.

Even more committed to the Syrian regime's survival are Iran and the Shia paramilitary movement Hezbollah in Lebanon. Both are highly conscious that the attempt to overthrow their long-term ally in Damascus is aimed at weakening them, and they are determined to repulse the threat. It makes sense for them to want to fight while Assad is still in power and not wait until he has been displaced by a hostile Sunni regime.

One important aspect of the Syrian conflict as it affects the US and Britain is lethally similar to the Iraq war. In each case any outsider intervening becomes involved in several inter-related but separate conflicts. In Iraq in 2003 the Iranians were glad to see the fall of their old enemy Saddam Hussein but fearful that an Iraq occupied by the US and Britain would provide a platform to overthrow the Tehran government. They did everything to make sure the Americans and British never stabilised their occupation of Iraq. "The Iranians have a PhD in this type of covert warfare," remarks an Iraqi specialising in national security.

So much of what US and British leaders or commentators say about Syria sounds phoney or unrealistic because they focus on only one of the four or five conflicts going on in the country as a reason for intervening. The struggle most often picked as a respectable motive for backing the rebels is the popular revolt against the brutal Syrian police state which started in March 2011. But this uprising swiftly became a sectarian war with the Sunni Arab majority pitted against the ruling Alawite Shia sect and other minorities, such as the Christians and Druze. The Syrian civil war is also part of the intensifying Sunni-Shia conflict that is being waged in the tier of countries between the mountains of Afghanistan and the Mediterranean. And this sectarian battle is linked in turn to the confrontation, dating from the overthrow of the Shah in 1979, between the US, Israel, Saudi Arabia and their allies, and an Iranian-led coalition.

If the Syrian political and military battlefield sounds very complex, it is; and it's getting worse. A savage ethnic war exploded in north-east Syria last month with the al-Qa'ida-linked al-Nusra Front and the Islamic State of Iraq and the Levant driving 50,000 Syrian Kurds into Iraq.

US and British leaders selling military intervention in Iraq and Syria seldom explained and often did not understand this mesh of conflicts. But these contradictory alliances determine the political map of the region and the reality of foreign involvement in it. It is easy, for instance, to advocate arming and protecting Syrian villagers whose children are being incinerated by napalm dropped by government aircraft. But what if those best able to help those villagers are the veteran jihadi fighters of the Islamic State of Iraq and the Levant, who have just chopped off the heads of Alawite prisoners and shot dead a teenager selling coffee for blasphemy? For all the disclaimers, US forces attacking the government in Damascus are in de facto alliance with al-Qa'ida. Likewise in Iraq 10 years ago, the US and Britain were pretending to be fighting for democracy and against the remnants of Saddam's regime. The reality was that in 2003-06 they had imposed an old-style imperial regime and had become participants in a cruel Sunni-Shia civil war on the Shia side.

What can be done to end the appalling and ever-growing miseries of the 23 million Syrian people? The answer is to make either war or peace effectively. Limited missile strikes on Syrian military bases are not going to compel President Assad to negotiate his own departure from power. The only military action that might do this is a full-scale assault including a no-fly zone and a no-drive zone. This means giving the rebels an air umbrella, as was done for the anti-Taliban Northern Alliance in Afghanistan in 2001, the Kurdish peshmerga fighters in northern Iraq in 2003 and the anti-Gaddafi militiamen in Libya in 2011. And thus fighting a full-scale war with the likelihood that Russia, Iran and Hezbollah will increase their support for Assad. Anything less than this full-scale military commitment simply stokes the war and increases the violence. It gives the opposition hope (particularly since so many of its official leaders reside safely abroad) that one day there will be a Libyan-type intervention by Nato on their behalf.

Limited intervention means that the stalemate will continue. One of the best chances for peace – the day of mutual exhaustion and realisation that nobody is going to win on the battlefield – is postponed. The analogy with Kosovo in 1999 is shallow and misleading since defeat was only admitted by an isolated Serbia after a 78-day air bombardment and the threat of a Nato land invasion.

If all-out war is not feasible, could peace come by negotiation? Here America and Britain's stance has been hypocritical, publicly supporting peace talks while offering only surrender terms to the Assad government at a time when it controls most of Syria. This was largely the result of a miscalculation by world leaders in 2011-12 whereby they underestimated the staying power of the Assad government. Its collapse was gleefully predicted, a role for Assad in Syria's political transition ruled out, while Iran, an important player, was to be excluded. A peace conference so out of keeping with the real balance of power is not going to stop any wars. But bringing Iran in would undermine the US, European and Israeli effort to isolate it over its development of nuclear power. The US would effectively have to recognise Tehran as a regional power, which would infuriate the Israelis and the Gulf monarchies.

Even then, peace would not come easily, if at all. The best interim solution could be a UN-monitored ceasefire as briefly occurred under the Kofi Annan plan in 2012. All sides are dependent on outside backers, and even those who most want to fight need weapons, ammunition and money. Heavy pressure could be put on them to agree to a peace conference and a temporary ceasefire.

This would be a Lebanese-style truce – unsatisfactory but better than full-scale war. A peace conference on this basis could be the political and diplomatic counterpart to the limited US military strike President Obama is contemplating. In practice there has been a stalemate in most of Syria for the last year. If the Syrian army did use poison gas, it shows it does not have the strength to retake even the inner rebel-held suburbs of Damascus. It is better therefore for the battle lines to be frozen under some form of UN supervision. Long-term solutions will only begin to be feasible when Syrians are no longer at the mercy of what Northern Ireland politicians used to call "the politics of the last atrocity".