Today's decision by Russia and China to veto a United Nations Security Council resolution to impose sanctions on the brutal regime in Syria is, at most, a hollow victory for President Bashar Assad. Russian officials say they opposed the measure for fear that it would lead to regime change, possibly with the assistance of western military forces, as in Libya. But given the events on the ground this week in Syria, the veto appears likely only to ensure that regime change comes through blood and chaos, not diplomacy.

The explosion at a military headquarters in Damascus Wednesday that killed at least three of Mr. Assad's closest aides left little doubt the rebels were capable of striking at the heart of his regime. It comes at the same time that rebel forces are advancing on other regime strongholds, as well as several days of some of the worst fighting the capital has seen since the uprising began 16 months ago. The killing or wounding of so many trusted members of the dictator's inner circle in a building where they expected to be safe clearly signals his once iron grip on power may be waning. When the end eventually comes, the U.S. needs to be prepared for the worst if, as appears likely, the country descends into full scale civil war.

The attack on a military intelligence headquarters in central Damascus, which a rebel spokesman quickly took responsibility for, claimed the lives of Mr. Assad's defense minister, Dawoud Rajha, his brother-in-law and military deputy chief of staff, Assef Shawkat, and Hassan Turkmani, a former minister of defense and Mr. Assad's security advisor. Mr. Shawkat, in particular, was known for the zeal and brutality of his efforts to crush the rebels. In addition to the deaths, a number of other high-ranking political, military and intelligence operatives were injured in the blast, including Mr. Assad's interior minister.

That such powerful officials could be targeted for assassination despite the heavy security presence surrounding the building where the meeting took place almost certainly points to an inside job. For Mr. Assad, that's an ominous sign. It means that from now on he and his family can't afford to trust anyone, regardless of how loyal they may have been in the past. It also raises the question of whether the government can continue to function under conditions in which everyone, including the security services on which the regime has long counted to stay in power, is now under suspicion.

That uncertainty has been intensified recently by the wave of defections that have taken place among Syrian military officers and rank-and-file soldiers alike. In recent weeks at least 20 Syrian generals reportedly have either gone over to the rebel side or fled to neighboring countries. News organizations reported fresh defections from the security services within hours of Wednesday's attack, leading to speculation the government could be in danger of imminent collapse.

Given his past actions, it's unlikely Mr. Assad will give up power unless that happens. He obviously has no regard whatever for the suffering he has inflicted on Syria's beleaguered civilian population, and while he insists that his stepping down would plunge the country into chaos and civil war, the truth is the longer he stays, the deeper the crisis enveloping his country will become.

Even so, Russia and China are refusing to pull the plug on this brutal dictatorship, and now the U.S. and its allies must prepare for the worst, including the possibility that a sudden disintegration of the regime could allow some al-Qaeda-backed elements of the opposition to get their hands on Syria's large stockpile of chemical weapons. Though the Obama administration has been right to be cautious about intervening militarily in Syria's upheaval, the possibility that radical Islamists might use such weapons to attack the West could lead the president to consider a narrow but prompt and decisive U.S. militaryresponse. Reluctant as the administration has been to jump into another Mideast war, such an action by the U.S. and its Western allies would be less destabilizing than similar action by Israel, and it may be the only way to head off what Secretary of Defense Leon Panetta described on Wednesday as the most dangerous scenario imaginable for a regime that is rapidly spinning out of control.