Thanks to the dogged work of Detlev Mehlis, United Nations chief investigator, the world now knows that the detailed planning for the assassination of Rafiq Hariri, Lebanese prime minister, in February was in the works for many months. However, according to one witness interviewed in the investigation, the final decision sealing Hariri’s fate was apparently taken by the highest Syrian and Lebanese intelligence officials only a week before, allegedly in the Damascus home of Assef Chawkat. That is right, the brother-in-law of ­Bashar al-Assad, the Syrian president.

Even though the Mehlis report is, in legal terms, an interim step on the road to possible indictments of senior Syrian and Lebanese officials, its findings have already created a political bombshell with profound ramifications.

The UN Security Council should now take the next step by ratcheting up the pressure on the Syrian regime. Be-cause the Mehlis report makes clear that the investigation needs to be pursued further and that Damascus is obstructing, it there is now an opportunity to pass a resolution that would extend Mehlis’s mandate and threaten sanctions if Syria does not co-operate quickly.

This will place the weak and maladroit Syrian president on the sharp horns of an irresolvable dilemma. To co-operate with the investigation may mean surrendering his brother-in-law to international justice – an unthinkable betrayal of family that for Mr Assad would entail the risk of a coup. To dismiss the demands of the UN Security Council, however, would subject his country to increased isolation and economic hardship and over time risk the increasingly tenuous hold on power of his minority Alawite regime.

Mr Assad has already sought a middle way out of this dilemma, sending emissaries to Washington to offer a Libyan-style “package deal”, involving the surrender of lesser officials and an end to Syria’s rogue activities. But his offer comes far too late.

President George W. Bush has already taken the measure of the man and found him unreliable. Mr Assad’s commitment to stop Syrian support for the Iraqi insurgency was honoured in the breach. His withdrawal of Syrian troops from Lebanon was followed by a bombing campaign that has forced many of the Lebanese political class to flee. Even people in Washington (like me), who once advocated a “carrots and sticks” approach to the Syrian ingénue, have given up on him.

Given Mr Assad’s weakness, and the thuggish nature of those who hold sway over him, we should expect ugly actions in response to international pressure rather than compliant ones. But Syria’s behaviour is now under an international microscope. Lies, obstruction of justice, sponsorship of terrorist attacks, assisted suicides and assassinations will only serve to tighten the noose around the regime’s neck.

In these circumstances, that odd couple of Mr Bush and Jacques Chirac, the French president, who are leading the international effort against Syria, need to tread carefully. They almost have the Syrian regime on the ropes and need to avoid missteps of their own. Fortunately, much seems to have been learned from the Iraq debacle. Wiser heads have tempered the instincts of those in the Bush administration who would seize the opportunity to attempt to overthrow the Assad regime. They understand that aggressive moves now risk losing the international legitimacy that Syrian blunders have conferred on American demands. Military action also risks promoting a descent into chaos in Syria, in which the Alawite regime and a Sunni Islamist insurgency engage in a bloodbath, compounding sectarian warfare in Iraq.

The better course of western action is to have the Security Council take the lead in demanding Syrian co-operation and threatening sanctions. That will require the US to restrict itself to an active behind-the-scenes role. For the more the Bush administration makes unilateral demands on the Syrian regime, the more others will feel obliged to come to its defence and resist robust Security Council action.

Western powers should remain patient and methodical as they bring the last rogue regime in the Arab world to account. Steps will need to be taken to deter Syria from further destabilisation of Lebanon and to prevent insurgents crossing into Iraq. But backing the judicial process launched by Mr Mehlis with the threat of sanctions and ensuring that it is allowed to reach its final conclusions is the best way to deal with the current opportunity. Mr Bush and Mr Chirac can count on the ­Syrians to do the rest for them.