In Halabja and then again in Srebrenica, political leaders who enjoyed pretending they were avuncular men of the world, displayed their true colors by ordering massacres as acts of terror. Saddam Hussein met the hangman’s noose for his crimes, as did his cousin Ali Hassan Al-Majid, or “Chemical Ali”, who actually planned and carried out the 1988 crimes. Radovan Karadzic, the Bosnian Serb president, is still on trial in The Hague for the slaughter of up to 6,000 men and boys in 1995.

What is important about the trials of these men is that, despite the horrific crimes that they had committed, and despite the often-harrowing evidence presented against them by the prosecution, the act of putting them on trial was unemotional. Though grieving relatives outside the courtrooms might have wished to tear them limb from limb and though many decent people would have sympathized with their white fury, the power of justice was that these men, obvious though their guilt may have been, were given the benefit of due process in a court of law. They could defend themselves against their accusers.

Even in their downfall and humiliation, when they were powerless prisoners, a dispassionate judicial system give them the chances that they all too happily denied their thousands of helpless victims.

This week another great crime has been committed. Maybe as many as 1,300 people died from asphyxiation, when poison gas shells were fired at their neighborhood in one of the outer suburbs of Damascus. This was an area of the capital, which the Free Syrian Army is fighting to hold in the face of heavy assault by the regime’s far better equipped armed forces.

There were undoubtedly opposition soldiers among those who died in the clouds of toxic gas fired into the suburb. But there were also civilians, who had discovered that the nightmare of street fighting had come to their part of the capital. Their sympathies may have been with the rebel fighters, but their main concern will have been the protection of their families, homes and businesses. They did not wish to join the already immense tide of refugees. So they stayed. And hundreds of them paid a terrible price for this decision.

There will come a day when some, at least, of the Assad regime’s leadership is captured after the dictatorship falls. Pumped-up rebel troops might want to administer summary justice there and then. It must however be hoped that the detainees will escape the same fate as Libya’s Muammar Qaddafi — dragged out of a storm drain, obscenely tortured and then shot dead on the spot.

There is a strong argument that had Qaddafi survived and been put on trial, perhaps best of all in The Hague, there would have been closure for the Libyan people, subjected as they had been to 43 years of ruthless and pitiless rule by a single man. Had the international court tried him, he would never have lost his life when found guilty. He would instead have moldered in a comfortable prison for the rest of his days. The rejoicing would have had to wait until news broke that he had died in his cell. Saddam’s eventual execution after a fair trial, gave a more immediate closure.

The UN inspectors currently in Syria to check reports of poison gas use should be allowed to visit the site of this week’s toxic attack in the capital. The government is denying it used the evil weapon. It therefore has no reason not to permit a UN inspection. Nevertheless, the chances of this being allowed would seem to be remote.

In truth, despite the many new levels of government violence which this conflict has reached, what happened in Damascus early Wednesday was a defining moment. Whatever tiny opening to negotiations, whatever the remote possibility of somehow including the dictator and some of his creatures in a provisional government, whatever outside chance that Assad could escape international justice by going into exile somewhere, each and every one of them disappeared when those gas shells landed among women and young children and spread out their deadly vapors.

The entire civilized world can now see what so many of us here in the Middle East have been protesting for the last two years — that Bashar Assad is acting as a terrorist against his own people. Washington has described the proven use of chemical weapons as the thin red line. Assad for sure just crossed it. So what are the Americans going to do now?