The discovery of the handcuffed bodies of more than 100 young men in a canal in Aleppo has ratcheted up calls for outside powers to act to stop the unrelenting violence in Syria. Although the circumstances of the mass murder are unclear, the opposition Syrian National Coalition has denounced the "extreme complacency" of most countries towards "the perpetrators of genocide".
In a globalised world, where outrageous abuses of human rights are everybody's business, the case for outside intervention in Syria ought to be convincing. Yet the history of armed interventions under a humanitarian banner is mixed, to say the least.
Two recent military operations - in Libya, to remove Muammar Qaddafi, and more recently in Mali, to turn back the jihadists who threaten to take over the country - might give the impression that western armed intervention is now the norm.
Yet history shows it is the exception. Even the strongest proponents of humanitarian intervention struggle to see how it might apply in Syria so long as the regime of President Bashar Al Assad has the support of Russia.
Not all interventions require a United Nations mandate. In 1982, a multinational force of US, French, Italian and British troops was stationed in Beirut to protect Palestinians following the massacre by Israeli-allied Falangist militiamen in the Sabra and Shatila camps. In 1991, following the first Gulf War to force Iraqi troops out of Kuwait, western powers stepped in to protect the Kurds, who were fleeing in their thousands from a vengeful Iraqi army.
But there are more cases where the world has turned a blind eye. The Rwandan genocide of 1994 was met with indifference, with the UN Security Council keen only to evacuate foreigners and reduce the presence of UN troops there.
The 1991-2002 Algerian civil war that may have killed 200,000 people was relegated to the news in brief columns of the foreign press. As for Bosnia, the worst conflict in Europe since the end of the Second World War, the European media in the early 1990s were filled with graphic horror stories and appeals for intervention. But it took three years and thousands dead before the Nato alliance attacked Serb forces from the air and forced them to lift the siege of Sarajevo.
All these conflicts have some lessons for Syria. In Beirut, it was clear that western powers had been guilty of abandoning the Palestinians. The multinational force had earlier overseen the evacuation of armed Palestinians from Beirut - leaving the camp residents helpless against their enemies - and then withdrew. The moral case that the withdrawal of the force had facilitated the massacre was unanswerable.
The same applied to the Kurds of northern Iraq. After the US-led coalition drove the Iraqi army out of Kuwait in 1991, the Kurds rose up against the regime of Saddam Hussein, believing they had the encouragement of Washington, but no US support was forthcoming.
The pictures from the mountains of northern Iraq of women and children fleeing to safety were irresistibly affecting, helping to force the hand of the coalition countries to impose a no-fly zone over northern Iraq. By contrast the Arabs of southern Iraq, who were in the same position as the Kurds, but isolated from the media and lacking the heroic backdrop of the mountains, were left to suffer their fate, although they did gain a no-fly zone a year later.
To justify armed force for a humanitarian end requires powerful media support, which in turn necessitates a simple media narrative of good versus evil. This was in evidence with the Kurds (Saddam Hussein) and in Beirut (Ariel Sharon and the Lebanese Falangists). But even when the press is clamouring for action, as in the Bosnia war of the 1990s (with Slobodan Milosevic as the hated figure), it took Nato three years to launch the air war that eventually forced the Serbs to join peace talks.
It also helps if the military goal is achievable from the air. This was the case in Bosnia and also in Kosovo 1999, when Nato forces launched a bombing campaign to save the Kosovar-Albanian population from genocide, reports of which are now recognised to have been highly exaggerated.
None of these conditions is visible with Syria. The western powers have already shown they cannot be motivated by guilt alone. It was after all the brutality of the Syrian security forces in Deraa which turned peaceful protest into an uprising.
Nor is there any simple narrative of good versus evil. Mr Al Assad cuts a poor figure for a dictator; he looks more like an incompetent lacking the political skills to maintain the regime he inherited. As for the rebels, they are divided and take little care of the civilians in the areas they control. Their claim to moral superiority is tenuous. Once the Assad clan is gone, there will not be peace, but most probably years of war between the victors.
The lessons of previous interventions are not encouraging. Boots on the ground in Beirut in 1982 led to mass attacks on US and French soldiers in 1983. Political careers are made from killing Americans, and that would be the game in Syria. As every seasoned general knows (when he is not defending his budget), war is a business with unpredictable outcomes.
This has become all the more clear with the invasions on Afghanistan and Iraq. As the British forces came to conclude in Basra, their presence in Iraq only made the situation worse.
It is even more apparent after the killing of Qaddafi. The Nato air campaign may have been quick and successful, but it has now help to spread instability throughout the trans-Sahara region.
The Assad regime has taken courage from the West's indecision. He has been given a clear message that the only trigger for western military intervention would be the loss of control of Syria's chemical weapons. His future is far from assured. But short of a chemical-weapons catastrophe, no B-52s are going to appear over Damascus to bomb him out of his presidential palace.
On Twitter: @aphilps