Update: On Thursday, after this editorial was published, Russia and China exercised their vetoes as permanent members of the Security Council to block the long-overdue resolution that would have increased the threat of sanctions against Syria. It was a destructive action that will make it much harder to save lives and bring and end to this bloody conflict. Susan Rice, the American Ambassador to the United Nations, and Sir Mark Lyall Grant, the British ambassador, showed appropriate anger in denouncing the vetoes.

For 17 months, President Bashar al-Assad of Syria has waged bloody war against his people from his redoubt in Damascus, convinced that he could weather the storm. A bombing on Wednesday that killed at least three of his most senior aides — including the defense minister and Mr. Assad’s powerful brother-in-law — shows that the war can reach deeply into the capital.

There were conflicting news reports about the cause of the explosion. State television reported that it was carried out by a suicide bomber, while the main armed opposition group said explosive devices were detonated remotely.

There is no condoning such tactics, and it is impossible to know whether the assassination is any kind of a turning point. The armed opposition is obviously making gains, but there is still no sign that Mr. Assad is planning to leave power. He has proved to be even more brutal than his father and could crack down even harder on the opposition. Reports that his government is moving its stockpile of chemical weapons and might be preparing to use them are especially alarming.