It was the imminent threat to civilians in Libya's second city, Benghazi, that clinched the argument at the UN for outside intervention. But with multiplying reports that the fight is on for Syria's second city, Aleppo, the signs are that even government air strikes will not spur a similar Western and Arab alliance into action. Morally, that has to be deplored. The parallels between the plights of the two populations are clear. But these are different times, and Syria is not Libya.

Russia and China are still resisting concerted UN action even at the level of sanctions; they believe intervention went further in Libya than authorised. The US is already in election mode, and President Obama will do nothing that might jeopardise a second term. France has a new, less intervention-minded government, and the UK is cutting its military. There was no rush to offer troops to support Kofi Annan's peace plan, which has remained seriously under-resourced.

Among the options that remain, the enforcement of a no-fly zone over Syria, or parts of it, to create safe areas might be the most attractive, but that would entail neutralising all Syria's still-substantial airpower first. It would also risk precipitating the country's fragmentation into enclaves and a proxy Sunni-Shia, Saudi-Iran conflict not so far down the line. The only other realistic course would be for the outside world to arm and train the opposition. This may already be happening in a modest way, which could be one reason why the opposition appears to be growing stronger.

But the perception that, after last week's attack on the security ministry, anti-Assad forces are in the ascendancy may offer the hint of another solution. In Libya, Muammar Gaddafi never seemed to acknowledge that his regime was doomed, and the West's messages about his future were mixed. As the US Secretary of State, Hillary Clinton, said pointedly this week, there is still time for President Assad to negotiate his departure. That time is short, but such an outcome would be infinitely better than a protracted civil war, and the crux is that no transition can begin until Mr Assad is gone.