A fork in the road for UK foreign policy
The prospect of parliamentary opposition preventing the UK joining in US military action over Syria is a defining moment in British foreign policy. It poses questions both about the future of the UK-US alliance and the ability of the British prime minister to make war without a majority in the House of Commons.
To be clear, there have been important moments before when a British government has declined to take part in a US-led campaign. Perhaps the two most significant were former Prime Minister Harold Wilson's decision not to commit British troops to Vietnam and John Major's to give the 1992 intervention in Somalia a wide berth.
There were differences of view back then between different government departments or Cabinet members. I can remember, for example, from talking to Whitehall figures at the time that the Ministry of Defence was hostile to the idea of deploying to Somalia, whereas the Foreign Office was more positive about the idea.
In the end though both the Wilson and Major governments made judgements, political ones, after hearing the views of various experts or interested parties.
Although the Wilson decision in particular was characterised by some at the time as a significant setback to the UK-US relationship there was no lasting damage and the two countries have happily joined forces to rain destruction on their foes in the years since.
Today's situation is different because it would appear that Prime Minister David Cameron is unable to deliver military support to the US despite his insistence that the two countries must stand together and that action must be taken to deter Syrian President Bashar al-Assad's regime from further use of chemical weapons.
Change in stance
From US President Barack Obama's point of view, this could transform the UK from the position of his country's most dependable ally into a significant complicating factor as he seeks to reassure a reluctant Congress and public that the US must act.
Mr Cameron's potential inability to deliver UK support reflects a failure to convince people, experts and public alike, that action against Syria is in the UK national interest. This is a particular irony given his 2010 electoral stance that British governments had sometimes been too ready to support the US without a proper consideration of whether it would benefit the country to do so.
Throughout the past two years, I have found contacts in the British military, Foreign Office, and intelligence services pretty much unanimous in opposing direct intervention in Syria, or in reciting all the practical difficulties that would prevent such action having a positive effect.
On Wednesday a senior military figure described to me the proposed limited cruise missile and airstrikes as "naïve and childlike", suggesting they would be too small to have any major effect on the regime and could produce all kinds of unintended consequences.
Do it "properly", with extensive support to the opposition over a prolonged period, or don't do it was the message - and it was given with a strong implication that there was no will in the US or UK to make a commitment of that kind.
It has been Downing Street that has repeatedly returned to the issue of armed intervention in Syria, albeit with occasional support from the Foreign Secretary, William Hague. But despite this determination, and the appalling images of suffering emerging from that country, neither the professionals in Whitehall nor the wider public have warmed to the case for getting more deeply involved.
So while Harold Wilson or John Major recognised that expert advice or public opinion were helpful in shunning a role in a US-led war, Mr Cameron tried to go against the grain and appears, barring a reversal of political fortunes, to have come unstuck as a result.
The dangers, in terms of White House views of Britain, of failing to deliver support are significant. It is already the case that many in Washington refer bitterly to Britain's failure to "stay the course" in southern Iraq or deliver a more positive outcome there.
There have been times when a British prime minister has managed to launch military operations despite public hostility or deep divisions. The gulf between public reluctance to get involved and a PM doing what he or she felt was necessary in order to maintain the UK-US relationship or Britain's permanent seat in the United Nations Security Council could be finessed or ignored.
This was a political reality, but also a constitutional matter, with Downing Street regarding itself as having "the Royal Prerogative", the ability to go to war without a vote of Parliament.
Under former Prime Minister Tony Blair this became increasingly hard to maintain, and of course, he recognised the importance of getting a Commons vote in favour of military action in Iraq.
At the time the Bush White House, seeing the political cost of doing so and potential for a humiliating defeat in the Commons, offered Mr Blair a way out. He declined to withdraw UK forces from the invasion.
Now it would appear that the ability of a British government to commit to military action without broad parliamentary support in any but the most urgent emergency has gone. Given the war weariness and indeed anti-Americanism of much of the British public, that has important implications for the future of UK-US relations.