Robin Niblett
Director, Chatham House

When David Cameron argued in September 2006 that the UK should henceforth be 'a solid but not slavish' ally to the US, last night's vote in parliament was not what he had in mind. It was the British Prime Minister who had pressed President Barack Obama to join the military operation in Libya and who forcefully advocated a more muscular approach to Syria than the US President has appeared to favour. The question was whether the US would follow the UK lead, not vice versa.

Now, the US must question the size and permanence of the gap that appears to have emerged over UK security policy between the British government, its public and, judging by the debate in parliament and media coverage, a large section of the political class and commentariat.

Strategic partners in international security

Since the end of the Second World War, the United States and Britain have been two of the countries that have best understood the importance of contributing to international security in order to sustain and benefit from international order. Indeed, it could be argued that the 'special' (or 'essential' as David Cameron and Barack Obama re-christened it in 2011) relationship rests on the bedrock of a shared approach to security that looks beyond the narrow confines of territorial defence and towards projecting security.

Both countries have made their share of errors in the sorts of interventions they have undertaken under this approach: from Vietnam to Iraq and the conduct of the campaign in Afghanistan after 2002. But there have also been notable correct decisions: from the first Gulf War to interventions in Kosovo, Sierra Leone, Mali and, in the case of protecting civilian life, at least, in Libya. And decisions not to intervene – in Rwanda, in Bosnia – have carried serious costs.

The US and the UK have also played a key role in projecting international deterrence. Together they deterred the Soviet Union during the Cold War, they deterred possible Iraqi attacks on its neighbours in the 1990s through to 2003, and they continue to deter Iran's recent attempts at regional hegemony.

However, in today's strategic environment, there are more threats to deter. The effects of violence and instability within countries now spread more easily across borders as states fail to provide adequate security, leaving vacuums for non-state actors including terrorist groups. And the US and UK have joined other nations in seeking to deter indiscriminate violence by governments against their civilians, as part of a growing international commitment to the 'Responsibility to Protect'.

But deterrence is only credible if transgressions are punished. Punishments can range from withdrawing aid to imposing sanctions to using military force. With very few exceptions, the US and UK have been side by side in advocating and using these tools across a range of crises in the past twenty years. Deterrence formed the basis for the US and British governments' plan to conduct limited and 'proportionate' military strikes against the regime of Bashar al-Assad, after Western intelligence services attributed a major chemical weapons attack in Damascus to the Assad regime.

Such a flagrant breach of the Chemical Weapons Convention should be unacceptable. Irrespective of the fact that the planned military strike would likely have little effect on the course of the civil war or on the suffering and death it is causing, conducting a military strike would serve notice on Assad and other leaders that the use of WMD in conflict remains forbidden. And it would remind such leaders that, in the absence of international consensus on how to enforce this, there is a group of nations committed to upholding this powerful norm in international affairs.

A more selective relationship

The rejection of the UK government's motion on Syria raises a major question mark over the UK's ability to continue playing its traditional role alongside the US in international deterrence. It also leaves the US in a more solitary position in having to shoulder the main burden of security in an increasingly insecure Middle East which is in the UK's and Europe's neighbourhood.

That being said, the US-UK security relationship will not fall apart as a result of the vote and the subsequent decision to rule out participation in a military attack on Syria. The bilateral security linkages run deep, from intelligence sharing and counter-terrorism to the nuclear relationship and close partnership in the UN Security Council and other international bodies. Shared US and UK strategic and security interests will persist in South Asia even after the withdrawal of coalition troops from Afghanistan in 2014, as well as in the Middle East. In fact, the UK may continue to contribute behind the scenes to US preparations for its expected punitive attack on Syria and to counter any potential spill-over from such an attack into neighbouring countries. 

But this is a time when, increasingly, questions are asked in the US about the UK's broader strategic posture. Deep cuts in UK defence expenditure have already had real impacts, as was seen in Libya. The UK is not closely involved in the most important dimension of US security policy for the future – the pivot towards the Asia-Pacific region. And the UK's growing ambivalence over its place in the EU means the US must cultivate other deeper bilateral relationships across Europe.

Philip Stephens in the Financial Times has described the United States under Obama as a 'selective superpower'. American policy-makers will now wonder whether the UK has become a more selective ally to the US. The element of doubt this inserts into the bilateral relationship is bad for both countries and for international security.