By Daniel Burke, CNN Belief Blog Co-Editor
(CNN) -- How did Syria go from an internal uprising to a wider clash drawing funding and fighters from across the region?
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 are using 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 obliterate 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.
"People hold onto religious fights longer than battles over land and water," said Nicole Bibbins Sedaca, an expert on foreign policy at Georgetown University and a 10-year veteran of the U.S. State Department. "It becomes existential and related to belief in a higher calling."
Some combatants in Syria appear to believe that fighting in the name of God justifies the most barbaric measures. Remember that video of a rebel eating the heart of a Syrian soldier while shouting "God is great!"? Or that horrific video showing the beheading of three men with butcher knives, also while praising God?
According to international reports and U.S. intelligence, Assad's regime has been just as brutal, killing at least 100,000 citizens, including hundreds in a sarin gas attack on Aug. 21.
As Congress holds hearings to weigh whether to respond to that attack with a military strike, Middle East experts say it's imperative to understand the major religious players in Syria, and why they are fighting.
The stakes couldn't be higher, experts say.
"If we come and and give one group a total win, we may be setting up an ethnic cleansing," Landis said.
The situation is Syria is fairly fluid, with lots of conflicting reports and shifting alliances, but here is our breakdown of the religious groups at war and a bit of background on their beliefs.
This small, secretive sect makes up just 12% of the Syrian population, but members have held prominent seats of power since the 1970s. Why? Because the ruling Assad family is Alawi.
Alawites consider themselves Muslims, but most mainstream Muslims call them heretics. Among the reasons: They believe that Ali, the Prophet Muhammad's cousin and son-in-law, is divine.
They've been ostracized almost since their 8th-century founding, so they keep many of their core beliefs secret. During the Ottoman Empire, they were not allowed to testify in court, Landis said.
"It was assumed they would lie, because the God they professed was man-made," he said.
In the 1970s, Hafez al-Assad, Bashar's father, built a brutal security force with fellow Alawites. They were the fingers of his iron fist.