The UK Parliament's decision not to intervene militarily in Syria marked an important watershed in UK defence and security policy. The consequences will be examined with interest by allies and potential adversaries alike.
Last night's defeat of the Government in the House of Commons was an assertion of Parliamentary sovereignty, on issues of war and peace, without modern precedent. It is now hard to see how any UK Government could undertake significant military action without the support of Parliament, or indeed of the wider public. And it is difficult to see such support being given unless there is a clear national interest involved, or if military operations are undertaken with the imprimatur of a UN Security Council (UNSC) mandate - at least until the shadows of Iraq and Afghanistan have faded much further from the national consciousness.
Some commentators have focused on the tactical errors and special circumstances that contributed to the defeat. Calling for a vote while the UN inspectors were still in Damascus was always going to be a very hard sell when the case for action rested heavily on an assessment of what happened on the ground. Government whips had limited opportunities to convince backbench sceptics, who had only just returned from their constituencies for this vote. The Prime Minister took Labour's support largely for granted, a surprising omission given the important role that Ed Miliband's opposition to the Iraq war had played in his surprise leadership victory in 2010.
The Shadow of Iraq and Afghanistan
Yet something more significant has happened. This was not a vote simply against the premature timing of the debate, or for greater consideration of evidence on whether chemical weapons were actually used (on which there can have been little doubt amongst MPs). Rather it reflects the reality - one that Cameron accepted as soon as the result was announced - that opposition to military action would have remained strong and widespread, whatever new evidence the UN inspectors eventually publish. For the experiences of Iraq and Afghanistan have left most MPs - and, even more so, a large majority of their voters - deeply sceptical of claims that military action can remain limited once the first shot is fired. The voices of those who speak of the consequences of inaction have, for now, been marginalised.
Many in the armed forces will welcome this decision. Over the last decade, their main operational focus has been to conduct operations - in Iraq and, after 2006, in Afghanistan - for which a strong basis of public support has been conspicuously lacking. This has never been a comfortable position for the armed forces of a democratic country to be in, and many will therefore be relieved by Thursday's vote.
No More Intervention?
The tide of interventionism had already ebbed substantially since its high-water mark, as evident by the Government's approach to Afghanistan and its evident reluctance to consider further 'boots on the ground' operations. But this decision marks a further step in that process. No UK government, for the foreseeable future, will be able to contemplate military action without first thinking about whether it is able to gain parliamentary approval.
Some of the military operations of the last two decades would probably still have gained approval from today's House of Commons. The liberation of Kuwait from Iraq in 1991, under a clear UNSC mandate, would likely have been overwhelmingly approved - as would the support that the UK gave to the US in the overthrow of the Taliban after 9/11.
But most other interventions of the last quarter-century would have found it hard to get past the sceptical gaze of the UK public as it is today, or of the House of Commons. With the benefit of hindsight, there is little doubt that they would have opposed the invasion of Iraq in 2003. But they would also have been sceptical of 'wars of choice' where the UK Government wanted to get out in front of the Americans - the UK's costly 'surge' in Afghanistan from 2006, certainly, but also the UK's support for the use of its own ground forces in Kosovo in 1999 (to the great irritation of Bill Clinton), and the UK / French led drive for military action against Gaddafi in 2011.
How much impact a further ebbing of appetite for intervention will have on the UK's relationship with the US remains an open question. For some in the US foreign policy establishment, this vote will be seen as further evidence of 'anti-Americanism' and wider European 'demilitarisation', as Richard Haass has commented in today's Financial Times. Yet they must surely be aware that the trends in UK opinion parallel similar developments in the US. President Obama still seems set on conducting limited strikes against Syria over the next few days. But he has shown little appetite for further military action, unless Assad chooses to escalate further or use chemical weapons on a large scale again. The most likely scenario still remains that the war will grind on, with horrific human consequences, and the West will not intervene again.
If the US finds itself involved in further significant military action in the Middle East (for example against Iran), it is now less likely that the UK will feel able to join it. But this vote will also add to the voices of those within the US, including President Obama himself, who are themselves weary of repeated military involvements in the Middle East.
The risks from this UK vote therefore lie, not so much in relation to the special relationship - which remains important and useful to both parties - as in what it says about wider trends in UK and Western willingness to use military force in future. The UK Parliament and public are no longer prepared to give their Government the benefit of the doubt on military operations, and the Government will be constrained in what it can do in future as a result. The consequences for UK defence and foreign policy will be examined with interest by allies and potential adversaries alike.
 Richard Haass, 'Britain drifts towards isolation', Financial Times, 30 August 2013.
Proliferation and Nuclear Policy