Despite the outrage at last weekend's Houla massacre, likely perpetrated by government forces and allied militiamen, military intervention in Syria remains unlikely.

Russia will not authorise even a UN arms embargo, let alone intervention. Syria's rebels look hopelessly divided and weak. And the rise of suicide bombings in Syrian cities has heightened fears for the safety of any intervention force.

The massacre, in other words, is not a game-changer. But what would be? There are least three potential triggers for Western involvement in a Syrian campaign.

First, Syria's large chemical weapons stockpile could be a flashpoint. Syria possesses hundreds of tons of VX, Sarin and mustard gas, much of it located close to restive areas.

Any sign that the Syrian government was at risk of losing control of these sites would set off alarm bells. Israel fears such weapons could fall into the hands of the Lebanese militant group Hezbollah, and has hinted that it would step in to prevent any such transfer.

Western states are more worried about al-Qa'ida in Iraq and other jihadist groups. The second trigger could be a change in Turkey's position. Turkey, which hosts the Free Syrian Army on its soil, has grown increasingly bellicose towards its former ally to the point of suggesting it might invoke Nato's self-defence clause in response to shelling across the border.

Turkey also wants to keep an eye on the Syrian opposition, and ensure that Kurdish rebels don't gain too much strength. If Turkey announced it was carving "humanitarian corridors" into Syria, it might change the American and European calculus, particularly if Arab states were open to contributing manpower.

The third trigger might be another highly visible massacre – but only in conjunction with a strengthening of the rebels' position.

President Obama is in the midst of a tightly-contested election campaign, and he has come under significant pressure from his political opponents for perceived prevarication in the face of mass killing.

Another incident like Houla might generate irresistible pressure from Democrats and Republicans alike. Western states, far from looking for a pretext to intervene in Syria, are hunting for every reason not to do so.

That will only change if the conflict lurches in new, unexpected directions.

Shashank Joshi is a research fellow of the Royal United Services Institute