Syrian rebel sources concede this. They admit that in the ferocious weeks-long struggle for control of the strategically important town of Qusair that lies between the Syrian capital of Damascus and the country’s third-largest city, Homs, al Nusra fighters are taking the lead. Likewise, they claim that there are few Syrian government forces now in the fight. “We are fighting Hezbollah,” one of the sources said via Skype.

Hezbollah aren’t the only ones battling on Assad’s side. Rebels claim Iran is deploying more fighters from the country’s al-Quds force, part of the Iranian government’s Revolutionary Guards.

And there is mounting evidence that Russia is keen to reassert itself in the Middle East and that the country has sent more military advisers into Damascus. That also raises the stakes, according to a Pentagon official, who spoke on condition his name wouldn’t be disclosed. As he put it: “What happens if we impose a no-fly zone in northern Syria and launch airstrikes on Syrian air force bases to make that happen and kill in the process some Russian military advisers?”

As he suggests, proxy wars can get out of hand. That is partly what happened in 1914 when the great European powers—Britain, France, Germany, Russia, and Austria—competed for geopolitical influence by backing rival nationalities and revolutionary groups in the Balkans. The consequence was the slayings of Archduke Franz Ferdinand and his wife, Sophie—and ultimately one of history’s deadliest conflicts.