AS 2012 COMES to a close, Syria is headed toward a bloody and chaotic end to what began as a peaceful uprising against an autocratic regime. This would be a catastrophe that could destabilize much of the Middle East, provide al-Qaeda with a new base of operations, and lead to the transfer or even use of chemical weapons.

Above all, the crisis is the result of the brutality and ruthlessness of ruler Bashar al-Assad and the family clique around him, and their supporters in Iran and Russia. But it is also reflects a massive failure of Western — and particularly American — leadership, the worst since the Rwandan genocide two decades ago.

The appalling consequences of non-intervention by leading nations in Rwanda led, after much soul-searching, to the adoption by the United Nations of the “responsibility to protect” doctrine, which provides for the international community to take action to stop crimes against humanity. Some of its leading proponents are senior officials in the Obama administration. But with the U.N. Security Council blocked from action by Russia and China, the administration has utterly failed to take or organize steps that might end the carnage in Syria. Instead, close allies of the United States, including Britain, France and Turkey, have watched with growing dismay as the White House has concocted excuse after excuse for passivity, most recently a claim that direct U.S. aid to Syrian rebel forces — or even to civilian organizations — would be illegal.

For much of the year, the administration embraced the feckless diplomacy of U.N. mediator Kofi Annan, along with the far-fetched prospect that Russia would support a negotiated transition that would replace the Assad regime with a coalition government. Mr. Annan eventually resigned, and now a new envoy, Lakhdar Brahimi, is pursuing the same scheme. Though some reports suggest that Moscow is finally ready to support the plan, the chances it will move forward are virtually nil. Rebel forces, which have been slowly but steadily gaining the upper hand on the battlefield with help from an al-Qaeda splinter group, are in no mood to negotiate with the regime. Mr. Assad himself appears prepared to fight to the death, and it’s doubtful that Russia could sway him even if it tried.

The most likely scenario is that rebel forces will, in a matter of weeks or months, win the war — or at least cause the Assad clique to retreat to its ethnic stronghold on the Mediterranean coast. If the world is lucky, this will happen relatively quickly, or an internal coup will remove Mr. Assad. If not, the bitter endgame could see tens or thousands more deaths and the use by the regime of its chemical weapons. Either way, the postwar scene in Damascus will likely be chaotic, with the Western-backed rebel coalition jockeying with al-Qaeda and remnants of the regime.

If that happens, the United States may find itself with little influence. Most rebel leaders, and average Syrians, are furious at Washington for withholding meaningful aid. They may be disinclined to listen to calls for dismantling the extremist groups that helped win the war. One way or another, Syria will haunt President Obama’s second term — and, based on the record so far, it will be recorded as one of his greatest failures.