This is all our nightmares rolled into one. An irrational and unpredictable US President, compared to a Mafia boss by the former head of the FBI, is in a global stand off with a militaristic Russian President presiding over a Mafia state run by fractious oligarchs. Come back Dr Strangelove, all is forgiven. Trump and Putin are worst kind of leaders in the worst kind of crisis in the worst kind of civil war – one which has been described by the BBC’s Middle East correspondent as “a world war in miniature”.

Theresa May has placed British forces under the effective control of an unstable and unreliable President in the opening shots in the new Syrian war. Russia has promised retaliation. She had no authority to engage British forces in this dangerous military adventure because she refused to seek parliamentary approval for it. At least Tony Blair won a vote in parliament in 2003 before joining in the Iraq war. Tory ministers like Jo Johnson have justified her behaviour on the abominable, pre-democratic doctrine of Royal Prerogative. Yet in our constitution it is only legitimate for a Prime Minister to act without parliamentary endorsement when Britain is directly threatened. This is manifestly not the case.

It is an abuse of her office to participate in this act of puerile militaristic machismo. Donald Trump's diplomacy has all the wisdom and subtlety of a 12-year-old in a playground scrap. His mid-week tweet, taunting Russia to “get ready” for his missiles because “they will be coming, nice and new and smart” was unspeakable. For the leader of the greatest military power on the planet to talk like Joe Pesci in “Goodfellas” is not just demeaning to a great country, it is helping destroy the entire post-war geo-political settlement, which was premised on law and rules-based international relations.

True, international law is often honoured more in the breach than the observance, but that doesn't make it valueless. Until very recently there was a general understanding that national leaders should try to behave like adults and seek a legal basis for their actions. These missile strikes against Bashir Assad's regime, which the UK cabinet pre-emptively endorsed on Thursday, are clearly illegal. Syria is a sovereign state. There’s no justification in international law for military actions against other nations, unless in response to a real and present threat, or unless such action is authorised by the United Nations Security Council. This was the problem in Iraq, where was no second UN resolution, and why some claim that Tony Blair is a war criminal.

Force is also said to be justified if it prevents an imminent humanitarian catastrophe, as in Kosovo in 1999. But this is dubious, and anyway doesn't apply in Syria which has been a bloodbath for seven years. The place where there really is an imminent humanitarian crisis, according to the United Nations, is Yemen. But we don't want to go there because we've been arming Saudi Arabia's assault on civilised values in that benighted land.

The absence of legal justification appears not to worry the White House, No 10 or the legion of armchair generals in the UK media. Trump has decided that international law ends when he opens his Twitter account. Might is right, and to hell with snowflake nonsense about proportionality, due process and negotiation. This is a man who thinks starting trade wars is an “easy win”. It is bully-diplomacy.

But we’re told we must punish Bashir Assad for using chemical weapons. Politicians and pundits are willing to put aside their reservations about Donald Trump just so long as he gives the Syrian leader a bloody nose. Oh, but we mustn't actually damage Assad, because we don’t want regime change in Syria for fear of something worse. Nor do we wish to prolong the civil war by supporting the bizarre collection of jihadist groups and other rebels fighting against the Syrian president. So we'll do something that doesn't actually do very much: couple of cruise missiles, damage a few planes, then home in time for tea.

Anyone who believes that Donald Trump, or Vladimir Putin for that matter, can be trusted to lead some consensual damage-limited, no-cost military exercise hasn't been listening. Trump is, as the former head of the FBI, James Comey, says in his new book, a frighteningly belligerent and unpredictable figure. “The loyalty oaths. The us-versus-them world view”, reminded him of a mob capo. “The lying about all things, large and small, in service to some code of loyalty that put the organisation above morality and truth”. Such is the Commander in Chief we’ve entrusted to wade into a civil war where Russia, Iran, Saudi Arabia, Hezbollah, and Israel are in a Mexican stand-off reminiscent of the finale of Reservoir Dogs.

Is this what Putin was hoping for, a war on all fronts? Probably not, but no-one really knows the mind of the former KGB boss. As Peter Pomerantsev says in his book, “nothing is true and everything is possible” in Putin's Russia. We don't know for certain that he was behind the Salisbury poisoning, or whether the Douma chemical attack is real or the false flag claimed by the Russian ambassador. But in refusing to co-operate on the sources of Novichok, and by vetoing successive attempts by the UN to conduct a proper investigation of chemical weapons in Syria, Putin has anyway placed himself beyond the pale. There is no doubt about Assad's use of chemical weapons – in fact, the Organisation for the Prohibition of Chemical Weapons OPCW accuses both Assad and the Daesh rebels of widespread use of chemical agents.

So, they may well be guilty men, but is that a reason to enlist in Donald Trump’s Wild West posse? We know enough about the US President, surely, not to let him lead us into trade negotiations let alone war. British voters seem to appreciate this even if much of the media doesn't. Jeremy Corbyn has been ridiculed for saying that we should deal with chemical weapons through international law rather than bombing raids. But he is essentially correct, in that the bonsai military action proposed is not going to stop Assad and risks sparking a wider conflagration that nobody wants.

The Treaty that originally gave rise to the ban on chemical weapon in 1925, was anyway the result not of military force, but a painstaking legal process, much like the Good Friday Agreement. The OPCW has had considerable success, ironically, in Russia. In November, the UK Ambassador to the OPCW, Peter Wilson, congratulated Russia for “the completion of the verified destruction of Russia's declared chemical weapons”. This left out non-declared chemical agents, but it was a significant step nevertheless.

Trump's war will only hasten the production of these agents, and bolster the domestic power of both Assad and Putin. Yet, there are many other ways to skin a cat, or isolate a pariah state. The wisest commentator on Middle East affairs, Ben Judah, proposes financial warfare, by closing London offices of the Russian state-owned VTB Bank that funds the Assad war machine. That's also the way to hit Putin where it hurts, by resorting to “banks not tanks”. It might lead to some collateral damage to the UK financial services industry, but better that than World War Three.