Since the outbreak of the Syrian protests, Washington has set its face against any overt military intervention in support of the revolutionary forces. Though there have been many anguished cries that "something must be done" to stop the slaughter, the Obama administration has had good reasons for steering clear.
After the debacle of Iraq, Western public opinion has no stomach for invading Arab countries, however brutal their ruling cliques. In an election year, the White House just wants the crisis to disappear. There is no diplomatic consensus either in the Arab world in favour of the use of outside force, or at the UN Security Council, where Russia and China defend the Assad regime against Western sanctions.
The divisions among the opposition, the absence of any convincing leader and concerns - perhaps overblown - about growing jihadist influence in their ranks all make it hard for Nato countries to pitch in as they did in Libya.
Finally, there is a clear difference between Libya and Syria. Libya was a case apart from its Arab neighbours, and the downfall of Muammar Qaddafi had little knock-on effect among them (though this is not true of Mali, to the south, where the Libyan blow back has helped to destroy a once thriving democracy).
Syria, by contrast, is intimately linked to all the countries of the Levant, and to Iraq and Iran. Regime change there is not likely to be a contained explosion. Rather, it would be the starting gun for a new round of proxy conflict between Iran and the Arab states of the Gulf, with the Sunni-Shia division leading to a new round of fighting in Lebanon.
Thus the hope in Washington has been for President Bashar Al Assad to step aside and a credible general - one chosen perhaps with the blessing of the Kremlin - to take his place.
Since the blast which killed three top regime officials in Damascus on July 18, the tone of public debate has utterly changed. The end of the Assad regime is accepted as a foregone conclusion. The possibility of military intervention now seems to be on everyone's lips, though often in a tone of weary resignation rather than gung-ho optimism.
Hillary Clinton, the US Secretary of State, predicted on Tuesday that the revolutionaries would soon be able to establish "safe havens" from which to launch further attacks, as happened with the Libyan opposition. If these words are to have any meaning at all, they must include a no-fly zone to guarantee the safe haven from attack by the regime's helicopters and fighter jets, which are currently protected by a Russian-supplied air defence system which is designed to deter Israel. That makes intervention a hundred times riskier than in Libya.
In Turkey, prime minister Recep Tayyip Erdogan is also talking of military intervention, saying that the presence of Kurdish fighters in Syria linked to the outlawed PKK guerrillas might require Ankara to use military force as it has done repeatedly in northern Iraq.
The Israelis, still theoretically in a state of war with Syria, have been talking about intervening to secure and destroy Syria's chemical weapons, though the sheer scale of this enterprise would require a major force, not a Hollywood-style team of special forces.
The idea of military intervention has received backing by a new paper from the Royal United Services Institute (Rusi) in London. Its director-general, Michael Clarke, writes that the current approach to Syria is becoming difficult and that "external intervention, in some form, is now significantly more likely".
Of course, intervention has been going on quietly for months. Russia is openly supplying weapons to Syria, while Iran is apparently offering advice on cyber security and tactics. The revolutionaries have received arms supplies through Turkey from backers in the Gulf, and the intelligence services of the US, France and Britain, and probably many more countries, are surely on the ground.
The Rusi report outlines three main options for outside military intervention: action to prevent the regime attacking civilian populations and to support the opposition with a view to hastening regime change; stabilisation of a post-Assad government, at its request, to prevent inter-factional violence; and humanitarian relief, probably under UN auspices, to help besieged populations.
Col Richard Kemp, a former commander of British forces in Afghanistan, sums up: "Planners will no doubt have already concluded that options exist that would allow US-led forces in support of the Syrian opposition and with the backing of Turkey, Saudi Arabia and other regional players to end Assad's violence."
Military officers have to plan for all eventualities. But still this is a frightening conclusion. Consider the forces ranged against a successful outcome: Iran, determined to maintain its strategic alliance with Syria, even to the point of using its Lebanese Hizbollah allies; Russia, with the efficacy of its air defence system at stake, no doubt keen to show it can down a few Nato jets.
And what about the allies? Are they agreed on who they want to take over the government of Syria? Is it desirable for there to be Muslim Brotherhood-led governments in both Damascus and Cairo, two of the historic capitals of the Arab world? And how could the allies counter the inevitable impression that they were working to an Israeli agenda?
Thomas Friedman, the New York Times columnist, has joined the intervention debate too: a Syrian transition to consensual democracy would require a "midwife" in the form of tens of thousands of US troops on the ground, he wrote recently. But that is never going to happen, so the most likely outcome is conflict for years to come.
Few would disagree with Friedman that the chances of a long civil war are high. But for the moment, there is no need to project outside military force on a conflict which itself is becoming more militarised by the day. Until the means and goals of intervention are clearly identified, and the chances of success are soberly weighed in the light of experience in Bosnia, Iraq and Afghanistan, those ideas for military intervention are best left on the planner's shelf.
On Twitter: @aphilps