Sooner or later, we'll have to go into Syria

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On Friday, the Prime Minister warned that fear of being sucked into another Iraq was distorting the discussion over Syria, where Bashar al-Assad's regime continues to massacre civilians, and is close to crossing western "red lines" on the use of chemical weapons. It is not hard to follow his reasoning. Tony Blair's triumph in Kosovo in 1999 paved the way four years later for the furiously controversial British participation in the removal of Saddam Hussein. Today, Mr Cameron must be asking himself whether the largely successful intervention in Libya will be followed by nemesis in Syria. He will also be conscious of the paradox of pre-emption: you will never be thanked for the evil you have prevented, but you will be made responsible for everything that follows an intervention. Given the stakes and the background, therefore, Mr Cameron's caution is understandable.

The better comparison, however, is not Iraq or Libya but Bosnia, where Serb "ethnic cleansing" raged uncheckedfrom 1992 to 1995. There, Western powers at first refused to intervene for fear of becoming embroiled in an ethnic morass of marginal strategic relevance. The result was to weaken the strong multi-ethnic consensus within the Bosnian government in Sarajevo, as loyal Croats, Serbs and moderate Muslims despaired of outside help, and either emigrated or retreated into passivity. Their places were often taken by more extreme elements, including jihadists from abroad. The radicalisation of the Bosnian side then became a staple of the argument against intervention. Even if Bosnia might once have been saved, it was too late. Eventually, Nato was forced to step in, and the killing came to an end, but the unique multi-ethnic Bosnia, which might have been rescued, was shattered beyond repair.

We see a similar pattern in Syria. It should not be forgotten that the rebellion against Assad began in March 2011 as a non-violent protest against the brutality of his intelligence services in the southern town of Deraa. The unarmed demonstrators were shot down by regime forces, but it was some time before armed resistance began, and many months before the first substantial Islamist presence was reported. The lack of effective Western intervention, however, narrowed the middle ground, as secular moderates fled the fighting or went to ground. Radical Islamists moved more to the forefront, many financially and logistically supported by politically conservative and religiously radical Gulf Arab monarchies. These are effective fighters, but a terrible scourge to any Christians, Alawites and Shias who cross their paths. As in Bosnia, the retreat of the moderates has become an argument against intervention. We now regularly hear that it is "too late" to save Syria.

Aagain, as in Bosnia, the failure to deal with an enormous crime on our doorstep is a serious threat to Europe's security. Sooner or later the regime, which has already tested the waters with small-scale use of chemical weapons – will move to their full-scale deployment with terrible consequences. We are turning the dictator's line that there is no alternative between him and the Islamists, anarchy and destruction, into a self-fulfilling prophecy.

The implications of this for the rest of the Middle East are not only profoundly depressing, but dangerous, because it will reinforce our association with anti-democratic forces in the region, be they conservative monarchies or hereditary "republics". Moreover, the regime in Damascus is the principal conduit for Iranian support to Hezbollah, which it plans to unleash on Israel and the west if Tehran's nuclear programme is attacked. Bringing down Assad will reduce the Iranian appetite for adventurism. The conflict is on the verge of reigniting the civil wars in Lebanon, and perhaps Iraq too, where Sunnis and Shias are watching the fates of their co-religionists with alarm. In the end, we are going to have to intervene.

Military action will be much harder than it was in the former Yugoslavia, where we supported the legitimate, government in Sarajevo. Today, the Syrian government has lost all legitimacy, but the opposition lacks coherence. That does not mean, however, that nothing can be done. The support of the United Nations is desirable but not essential. As in Kosovo, the threat of a Sino-Russian veto should not stand in the way of decisive action to prevent further massacres, and stabilise the south-eastern approaches to the EU and North Atlantic Treaty area.

Britain should begin by asking Nato to impose a no-fly zone over the country, and threaten air attacks on all of the regime's heavy weaponry. This should be accompanied by a political initiative to support the opposition under the Syrian National Council to setting up free zones along the Turkish border, in which a provisional government should be established with representation from all major population groups. This approach needs to be embedded in a broader Western strategy to protect Middle East minorities, especially the Shias of Bahrain and Saudi Arabia whom we have neglected for too long. The west should insist (as in Bosnia) that foreign fighters be sent home, and local Islamists disarmed. Nato should then train and equip the remaining opposition forces until the regime has fallen. There is no guarantee that Christians, Alawites and Shias will not suffer reprisals, but there seems to be little support for this among ordinary Syrians, and in any case the biggest long-term threat to minorities' survival is the Sunni anger the regime is stoking.

Mr Cameron's timely intervention in Libya averted a massacre, and enabled its Western and Eastern halves to continue to live together, however uneasily. In Syria, by contrast, the prospects for coexistence become grimmer every day. Understandably, the Prime Minister has hesitated before taking action in Syria, but the longer he leaves it, the harder it will be.

Brendan Simms, professor of the history of international relations at Cambridge University, is author of 'Europe: The Struggle for Supremacy, 1453 to the Present'