Opposition 'took a look at three actors who had been pummeling them for years: Iran, Russia, and Hezbollah. They had seen them weaker and more exposed than before,' says National Security Adviser Sull

Diminished support for the Syrian government from Iran, Hezbollah and Russia enabled opposition forces to take the initiative and seize new territory.

Syrians are generally repelled by extreme religious rhetoric. As jihadist groups came to dominate the rebellion after pro-democracy demonstrations were crushed after the first year or so of war after 2011, many Syrians either stayed neutral or sided reluctantly with the regime because they feared the murderous jihadist ideology of Islamic State.