Because it would take time to establish such a tribunal and because there is an urgent need to stop Syrian forces from committing more crimes, the Arab League could specify that prosecutions for crimes committed after the resolution’s adoption would have priority. That would put the forces of Syria’s president, Bashar al-Assad, on notice that the surest way to end up in the dock is to persist in the crimes they have been committing.

We should not grant them impunity for crimes committed up to now. But the urgent need to prevent further atrocities justifies giving them an incentive to stop. Of course, some of those responsible for crimes would imagine that they would never be apprehended and brought to justice. Yet the record of other international tribunals makes it increasingly necessary for them to take such courts seriously.

Something similar took place during the Bosnian war, which began 20 years ago this week. Neither the administration of President George Bush nor that of President Bill Clinton was ready to intervene militarily. But both expressed outrage at the crimes of ethnic cleansing in that conflict. That led to American support for the establishment of what became the International Criminal Tribunal for the Former Yugoslavia. At the time, few took it seriously.

It had no capacity on its own to get hold of those accused of crimes. Hardly anyone imagined that the leading perpetrators could be brought to justice. Yet the court has been remarkably successful. Of the 161 people on all sides of the Balkan wars whom the court indicted for war crimes, crimes against humanity and genocide, all were eventually apprehended and brought to The Hague except those who died or had their indictments withdrawn; 64 were convicted and sentenced, and 13 were acquitted. The rest are appealing their convictions, are still on trial, have died or have had their cases referred to courts at home.

Indeed, national courts in Serbia, Croatia and Bosnia have conducted scores of high-quality trials of lower-ranking defendants accused of war crimes.