Since the outset of the Syria crisis in March 2011 there has been little appetite for outside military intervention. This has been based on two assessments.
Firstly, that the situation on the ground in Syria is in many ways very different from that in Libya - the opposition is much more divided, the government's security forces are much stronger, and Syria's air defences are more effective.
Secondly, there has been a view that the implications of toppling President Bashar al-Assad could prompt a much wider wave of instability in the region.
Unlike Libya, Syria - both politically and geographically - is a central player in the Arab world, and sectarianism and instability there could threaten both Lebanon and Iraq.
Then, of course, there is the fundamental legal problem. Constrained by Russian and Chinese vetoes at the UN Security Council, there is no possibility of getting a resolution to authorise force.
That has not always mattered in the past. Nato troops went into Kosovo, after all, to halt systematic abuses by Serbian forces.
But the absence of legal authorisation certainly precludes action when there is little enthusiasm for it in the first place.
So what are we to make of calls from senior Republican politicians in the US, like Senator John McCain, urging air strikes against Syrian security forces?
Joshua Landis, director of the Centre for Middle East Studies at the University of Oklahoma, says: "Despite the growing chorus of politicians calling for US leadership in Syria, the Obama administration is adamant that Washington should not take the lead, but follow regional partners, Saudi Arabia and Turkey."
Mr Landis argues that the simple fact is that the Obama administration sees no strong reason to intervene.
"US officials are unanimous in arguing that the Assad regime is doomed and can only hang on for a limited time, with or without increased US support for the Syrian opposition. I think they are right in this analysis."
"This means that the US has no compelling national security interest in jumping into the Syrian civil war that is emerging. The regime's days are numbered."
Much of the debate on outside intervention is vague. It confuses and makes false distinctions between the different options and to a large extent glosses over many of the fundamental problems facing them all.
Humanitarian focusAssistance and relief
The main thrust of any external action would be essentially humanitarian in nature, a response to the growing plight of civilians in Syrian towns and cities who are under bombardment by government forces.
Efforts could also be made to bring assistance to displaced refugees who have moved towards Syria's frontiers with Turkey and Lebanon. Three related measures are being discussed.
Humanitarian corridorsSuggested first by the French Foreign Minister Alain Juppe last year, the idea would be to establish short corridors into Syrian territory through which humanitarian supplies could be delivered.
The establishment of safe areas within Syrian territory is an idea that has been broached by the Turkish Foreign Minister, Ahmet Davutoglu. Such safe havens would be in border areas, acting as a place of safety where refugees could gather, be fed and sheltered, and so on.
Often discussed in relation to both of the above, the suggestion is that Western air power could help ensure the safety of the zones or corridors.
In itself air power would not halt the fighting. It is not the Syrian Air Force that is primarily involved in offensives on opposition strongholds - although activists say jets and helicopters have been used to carry our reconnaissance and occasionally air strikes - but the government's ground forces.
But a discussion of air power underscores that unless the Syrian authorities agree to allow humanitarian access, any safe zones or corridors would first have to be established by force and then defended by force. The horror of Srebrenica in the Balkans showed what can happen if there is no will to defend a safe haven once created.
The key point here is that even the options directed primarily at humanitarian relief require significant military elements too. They involve seizing and holding bits of Syrian territory and a willingness to take on Syrian forces if necessary.
Air power would be crucial - though Western warplanes would have to contend with Syria's reasonably sophisticated air defences. Boots would almost certainly be needed on the ground too. These are not "military lite" options.