The US and Britain are facing a range of pitfalls in their attempts to respond to chemical weapons being used on Syrian civilians.
Britain should avoid joining the US attack against Assad
A day can be a very long time in politics. On Wednesday morning, the British prime minister, David Cameron, seemed assured of getting parliamentary approval to join the Americans in a missile strike on Syria. He had called an emergency session of parliament that was being billed as the high point of his term as prime minister.
The prime minister sought an authorisation to join the Americans in punishing the Syrian regime for the alleged use of poison gas in the eastern suburbs of Damascus in which about 1,000 people died.
The timing was tight. The US president, Barack Obama, wanted to conclude the missile attack over the weekend before he sets off on Tuesday for Europe and then to a summit meeting in Russia of the Group of 20. He could then present the host, President Vladimir Putin who is the key backer of the Syrian regime, with a fait accompli.
But as the hours ticked by on Wednesday, Mr Cameron lost control of the issue. The opposition Labour Party, which had been in favour of authorising a strike, withdrew its support at 5:15pm. Its leader, Ed Miliband, said he would not give the prime minister a "blank cheque" for military action before the United Nations weapons inspectors in Damascus had reported back to the Security Council. This would delay the vote until next week.
The prime minister was forced to back down and agreed to a two-stage voting process, the second part of which - the operative vote on military action - may never take place.
Technically, a British prime minister can proceed with military action without asking for the support of parliament. But in this case, it was politically impossible. A growing number of MPs in his own Conservative Party were likely to revolt, unconvinced of the need to hurry through the decision to be directly involved in Syria's civil war.
The political drama in London would be quickly forgotten, but it underlines the contradictions in western policies towards Syria. There is wide agreement among America's allies that the use of chemical weapons cannot go unpunished. But there is strong resistance to the idea of getting involved in another military adventure which, given the results of interventions in Iraq, Afghanistan and Libya, is unlikely to bring peace.
The parallel with Iraq in 2003, when the US and Britain went to war to remove Saddam Hussein on the false premise of him having weapons of mass destruction, colours the judgements of many British MPs. The situation this time is reversed: Washington is not preparing to remove Bashar Al Assad by force. Mr Obama has declared the purpose of the retaliatory strikes to be a "shot across the bows" to prevent the use chemical weapons again.
A more apt comparison is the 1998 Anglo-American strike against Iraq known as operation Desert Fox, a four-day bombing campaign that was similarly designed to teach Saddam Hussein a lesson. The Iraqi leader absorbed this slap in the face and came to believe that he could survive whatever the Americans threw at him. Five years later, the allies launched a full scale invasion.
So the issue is not whether the US or Britain are about to put boots on the ground in Syria this year or next. The issue is that the paltry "shot across the bows" will mark the start of an open-ended engagement that will only get deeper.
These concerns are not restricted to Britain. In Washington, the speaker of the House of Representatives, John Boehner, has asked the president for a "clear, unambiguous explanation of how military action - which is a means, not a policy - will secure US objectives and how it fits into your overall policy". This is impossible for the president to answer. As his army chiefs have said, there are no positive outcomes imaginable from direct intervention in Syria.
Whatever Mr Obama does, he cannot satisfy his critics. By launching a few cruise missiles - perhaps with the support of France and even Britain if the timescale is allowed to slip - and then declaring the job done, he will fail to make an impression on a regime that is determined to fight to the death, and will fail to convince those who demand a plan to prevent a region-wide explosion.
What started as a small conflict is now being compared to the First World War, a parallel reinforced by the use of poison gas that was first deployed at the battle of Ypres in 1915. Zbigniew Brzezinski, national security adviser to president Jimmy Carter, recalled this week that the Great War began, as in Syria, with "initially trivial violence". Mr Brzezinski has called into question Mr Obama's reliance on military support from France and Britain. A policy of force based on former colonial powers, he says, "does not seem to me a very promising avenue to an eventual solution".
Inevitably, a solution to the war will require support from Russia. In 1999, it was Moscow that helped to end the Kosovo wars by convincing its ally, Slobodan Milosevic, the president of Serbia, to withdraw his army. These days Moscow's help will not be given so cheaply.
Many British politicians worry indeed how they will be seen in the region, not just as an aggressor, but as the eternally faithful squire to Washington's rather confused knight.
The British prime minister will not thank the parliamentarians for spiking his attempts to intervene in Syria according to an American timetable. Washington will look on Britain as a less reliable ally. But in the end, lacking clarity about the nature of the original chemical attack and the wider purpose of the intervention, it is better for Britain to stay out.
On Twitter: @aphilps