The situation in Syria is dire. After 20 months of conflict, the war has created a human and economic disaster. Around 40,000 people have been killed, many more injured and hundreds of thousands displaced. At the same time, media sources in late 2012 have reported major rebel advances and the withdrawal of staff from Syria by the United Nations and several diplomatic missions. All this, now reinforced by concern over Syria's chemical weapons, is creating an upsurge in support (especially in western Europe) for western military intervention, on the grounds that the moment could become the "tipping-point" for Bashar al-Assad's regime.

There is a clear need to assess the risks and probable consequences of such a course, and to examine the prospects for any sort of diplomatic solution. The context for both is the way the Syrian conflict has evolved since spring 2011, when Damascus reacted with great violence to the localised outbreaks of non-violent protest. Syria's power-elite drew from Tunisia and Egypt the lesson that it had to be ruthless in its repression and offer little in the way of concession. But ever more force against the eruptions of dissent only hardened the emerging opposition, and by mid-2012 a rebellion was developing. As this intensified in the autumn, many analysts doubted that the regime could survive the year.

It did survive, fortified by support from sections of the population, and despite a number of defections from its core. The conflict was evolving rapidly into a form of "double-proxy" war that, by involving regional and global actors, hugely complicated the search for a peaceful resolution. In the Middle East, the rebels were increasingly encouraged by Saudi Arabia and Qatar, with the Saudis in particular determined to see the regime fall. The Assad regime was strongly backed by Iran; weapons and training resources flowed in, greatly aided by an "air bridge" that transited Iraq (thanks to Nouri al-Maliki's government in Baghdad, in an act dismaying to the United States – see "Iraq, Iran, Syria: triangle of war ", 9 August 2012).

In the global arena, Washington and its allies were on the side of the rebels, while the Russians and to an extent the Chinese stood by Damascus. A further complication was the growing presence of Islamist paramilitaries, many of them travelling from elsewhere in the region. They proved particularly effective thanks to their intense commitment and motivation, but also because some had gained combat experience in urban warfare in Iraq. They also benefited from persistent disunity among the rebels, something only marginally less visible after the meeting in Doha in November 2012 which resulted in the formation of an inclusive coalition.

A new momentum

This double-proxy war has now reached a point where active western involvement is looking ever more likely. So far, western powers have confined themselves to channelling aid to "acceptable" rebels while trying to prevent Islamist groups from acquiring weaponry. An extension of this stance might initially take the form of supplying the rebels with more effective arms and erecting a "no-fly zone". Both are feasible short-term actions, although the latter could be made more difficult by the presence of 2,000 Russian advisers in Syria (see Gideon Rachman's The perilous drift to intervention in Syria, Financial Times, 4 December 2012). In this context Nato's decision to ship long-range Patriot missiles to Turkey is significant, not least because these could be used to help enforce a no-fly-zone over much of northern Syria.

The supporters of intervention have two cogent arguments: that an even worse humanitarian disaster must be prevented, and that a quick end to Assad's regime will diminish the risk of Islamist influence in a post-Assad Syria. They point to the evident increase in the number of Islamist-linked paramilitaries active in the conflict, with the Jabhat al-Nusra group alone claiming (a probably overblown) 10,000 fighters (see Samer Araabi, Despite growing violence, Syrian political equation unchanged, IPS/TerraViva, 5 December 2012). In parallel, the tactics of many rebels have become far harsher now that they have deprived the regime of the near-monopoly of terror it enjoyed in the conflict's early months; this has lost them support among some Syrians with no love for the regime.

These two propositions are reasonable, but leave three other issues out of consideration. First, any western military action will provoke Tehran into increasing its support of Damascus (which Baghdad may facilitate). Second, the fall of Assad's regime may turn out to be a prolonged process involving even greater loss of innocent life. The power-base in Syria still has domestic support, and far more military capability than Gaddafi's Libya (which survived six months of extensive Nato action).

Third, the wider impact of yet another western intervention in the Middle East may be disastrous. Assad's regime may be hated across much of the region, but it is still the government of a major Arab country. After all, Saddam Hussein was despised when the coalition's war to overthrow him began in March 2003; just a few months later, those who expected to be garlanded as liberators were widely seen as occupiers and even oppressors. This historical experience is usually ignored or its relevance dismissed – but it is vital, and it has to be faced.

A different endgame

The predicament over Syria remains appalling. The consequences of intervention could (as so often in the past) be unexpected and counterproductive, but to do nothing may allow even more blood to be spilled. There is, though, one possibility that could avert the worst outcomes: a decision by President Obama's administration – hitherto very cautious about intervention – to make a very strong effort to achieve negotiated regime change (see Syria: war and diplomacy, 23 August 2012).

Such a course would require the full co-operation of Moscow, which is not unimaginable: there are signs in recent days that Russia is seriously concerned about the regime's viability and what might follow its collapse (a sentiment that may even be shared in some quarters in Tehran). Amid all the violence and bloodshed, this provides a small window of opportunity; but it can be opened only if the US takes the lead in an intense diplomatic process that accepts the need for substantial – and uncomfortable – compromise over the shape of a post-Assad Syria.

The west, to put it bluntly, is not in a position to dictate what form Syria's evolving governance might take. It has to recognise that this must principally be decided within Syria – but that the acquiescence of other states in the process will be essential: Russia and Iran, but also Turkey and Egypt (independently of concerns over President Mohamed Morsi's domestic actions).

The lone hopeful element in this scenario is that Obama's re-election gives him room for action. Over Syria – as over Iran and Israel-Palestine – he could in principle follow a more considered approach, avoid the risks of escalating conflict, and seek the best possible solution available in difficult circumstances. Where Damascus is concerned, there is still a chance of some kind of arranged regime change – very tough though it would be to reach. Will that chance be taken? The answer lies mainly in Washington, but not a little too in Moscow and Tehran, and in Ankara and Cairo. The fate of Syria, and more than Syria, is in the balance.