Washington ? One of the worst recurring features of U.S. foreign policy is a process that might bluntly be described as “seduction and abandonment.” Now it’s happening in Syria.

The seduction part begins with an overeager rhetorical embrace. Nearly two years ago, on Aug. 18, 2011, President Obama first proclaimed “the time has come for President Assad to step aside.” He didn’t back up his call for regime change with any specific plan, but this hasn’t stopped him from repeating the “Assad must go” theme regularly ever since.

The next stage is a prolonged courtship with ever-deeper implied promises and commitments. The CIA began working with the Syrian opposition in 2011, and has been providing training and other assistance. When the Syrian opposition was wooed by other suitors (say, Turkey and Qatar), the United States chased those rivals away with renewed avowals of affection.

Then comes the formal engagement. On June 13, the White House announced it would provide military aid to the Syrian opposition because the Assad regime had crossed a “red line” by using chemical weapons. The rebels began preparing warehouses to receive the promised shipments — hopeful that at last the United States was serious about its intentions.

And then? Well, this is a story of unhappy romance, so you know what comes next. It’s what 19th-century English novelists called “the jilt.” To quote a New York Times story published last weekend, it turns out “that the administration’s plans are far more limited than it has indicated in public and private.”

Imagine for the moment that you are a Syrian rebel fighter who has been risking his life for two years in the hope that Obama was sincere about helping a moderate opposition prevail not just against Assad but against the jihadists who want to run the country. Now, you learn that Washington is having second thoughts. What would you think about America’s behavior?

Let me quote from a message sent by one opposition member: “I am about to quit, as long as there is no light in the end of the tunnel from the U.S. government. At least if I quit, I will feel that I am not part of this silly act we are in.” A second opposition leader wrote simply to a senior American official: “I can’t find the right words to describe this situation other than very sad.”

An angry statement came this week from Gen. Salim Idriss, the head of the moderate Free Syrian Army. After Britain, like the U.S., backed away from supplying weapons, he told the Daily Telegraph newspaper: “The West promises and promises. This is a joke now. … What are our friends in the West waiting for? For Iran and Hezbollah to kill all the Syrian people?”

What’s happening in Syria isn’t a pretty sight, as the moderates struggle to survive without the expected Western aid. Last week one of Idriss’ commanders, Kamal Hamami, was gunned down in Latakia by extremists linked to al-Qaida. This week, the same extremist group overran a Free Syrian Army warehouse just south of the Turkish border. Having spent hundreds of billions of dollars to stop al-Qaida in faraway Afghanistan, you might think the U.S. would try to check the terrorist group in Syria, but no.

The moderates are trying to hold on as the country crumbles. In the Bustan al-Qasr neighborhood of Aleppo, a Free Syrian Army commander named Abdel-Jabbar Akidi has tried to prevent extremists from blockading food supplies to civilians who have supported the regime. He’s also trying to stop a war between rival Shariah courts in the northern suburbs of Aleppo. This is a commander who has been pleading for almost two years for serious help from the West, apparently in vain.

The story that’s playing out now in Syria is so familiar that it’s almost a leitmotif of American foreign policy. Washington wants to see a change of government so it encourages local rebels to rise up. Once these rebels are on the barricades, policymakers often get cold feet, realizing that they lack public support. This process happened in the Hungarian Revolution of 1956, the Bay of Pigs in 1961, the Prague Spring of 1968, the contras program in Nicaragua in 1984. It happened in Lebanon, Laos, southern Iraq … make your own list.

At the end of 19th-century novels, the seducer who abandons his flirtation usually gets what he deserves: He is shamed and ultimately ruined, while virtuous and steadfast characters are rewarded. But it doesn’t happen that way in foreign policy.