Mitt Romney and congressional Republicans are doing their best to portray the assault on the U.S. mission in Libya and its aftermath as a signal foreign policy disaster for Barack Obama. But my bet is that when historians look back on Obama’s mistakes in the last four years, they will focus on something entirely different: his catastrophic mishandling of the revolution in Syria.

The deaths of Ambassador Chris Stevens and three other Americans in Benghazi were a calamity — but those losses were mainly the result of poor security decisions by mid-level State Department officials, not policy choices by Obama. The president’s handling of Syria, on the other hand, exemplifies every weakness in his foreign policy — from his excessive faith in “engaging” troublesome foreign leaders to his insistence on multilateralism as an end in itself to his self-defeating caution in asserting American power.

The result is not a painful but isolated setback, but an emerging strategic disaster: a war in the heart of the Middle East that is steadily spilling over to vital U.S. allies, such as Turkey and Jordan, and to volatile neighbors, such as Iraq and Lebanon. Al-Qaeda is far more active in Syria than it is in Libya — while more liberal and secular forces are turning against the United States because of its failure to help them. More than 30,000 people — most of them civilians — have been killed, and the toll mounts by the hundreds every day.

Of course, Obama is not solely responsible for this mess. But his serial miscalculations have had the consistent if unintended effect of enabling Syria’s Bashar al-Assad — first to avoid international isolation, then to go on slaughtering his own population with impunity.

Obama’s Syria policy began in 2009 with the misguided idea of reaching out to the dictator. Within a month of his inauguration, Obama reversed the Bush administration’s approach of isolating Assad. He later reopened the U.S. Embassy and dispatched senior envoys, such as George Mitchell.

The problem with this policy was not just the distasteful courting of a rogue regime but the willful disregard of the lessons absorbed by George W. Bush, who also tried reaching out to Assad, only to learn the hard way that he was an irredeemable thug. Yet Obama insisted on reversing Bush’s policy of distancing the United States from strongmen like Assad and Hosni Mubarak — a monumental miscalculation.

When the uprising against Assad began in March of last year, the administration’s first reaction was to predict that he could be induced to coopt it. “Many . . . believe he’s a reformer,” said Secretary of State Hillary Clinton. That illusion caused the administration to stand by for months while Assad’s security forces gunned down what were then peaceful pro-democracy marchers; not until August 2011 did Obama say that Assad should “step aside.”

By then Syria was already tipping into civil war. The State Department’s Syria experts recognized the peril: If Assad were not overthrown quickly, they warned in congressional testimony, the country could tip into a devastating sectarian war that would empower jihadists and spread to neighboring countries. But Obama rejected suggestions by several senators that he lead an intervention. Instead he committed a second major error, by adopting a policy of seeking to broker a Syrian solution through the United Nations. “The best thing we can do,” he said last March, “is to unify the international community.”

As countless observers correctly predicted, the subsequent U.N. mission of Kofi Annan was doomed from the beginning. When the White House could no longer deny that reality, it turned to an equally fantastical gambit: Vladi­mir Putin, it argued, could be persuaded to abandon his support of Assad and force him to step down. The nadir of this diplomacy may have been reached on June 30, when Clinton cheerfully predicted that the Kremlin had “decided to get on one horse, and it’s the horse that would back a transition plan” removing Assad.

Needless to say, Putin did no such thing. The war went on; thousands more died. For the past three months, Obama’s policy has become a negative: He is simply opposed to any use of U.S. power. Fixed on his campaign slogan that “the tide of war is receding” in the Middle East, Obama claims that intervention would only make the conflict worse — and then watches as it spreads to NATO ally Turkey and draws in hundreds of al-Qaeda fighters.

No doubt it’s easier for Romney and the Republicans to talk about the death of an ambassador in a terrorist attack than to ask war-weary Americans to think about this. But it is Syria that is Obama’s greatest failure; it will haunt whomever occupies the Oval Office next year.