In a word, Middle East experts say, religion.
Shiite Muslims from Lebanon, Iraq and Iran have flooded into Syria to defend sacred sites and President Bashar al-Assad's embattled regime. Sunni Muslims, some affiliated with al Qaeda, have rushed in to join rebels, most of whom are Sunni.
Both sides use religious rhetoric as a rallying cry, calling each other "infidels" and "Satan's army."
"That is why it has become so muddy," said professor Joshua Landis, director of the Center for Middle East Studies at the University of Oklahoma. "The theological question has returned to the center."
That's not to say that the warring parties are fighting over, say, the definition of God.
But the United Nations, in a series of reports, has warned with mounting urgency that the battle lines in Syria are being drawn along sectarian - that is, religious - lines. Both sides fear that whoever wins power will wipe out the loser.
"The conflict has become increasingly sectarian, with the conduct of the parties becoming significantly more radicalized and militarized," the UN said earlier this year.
And that's a really bad thing, foreign policy experts say.
Religious civil wars are longer and bloodier than other types of clashes, according to studies. They are also twice as likely to recur and twice as deadly to noncombatants.