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 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.
"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 the other 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 determine a response to that attack, 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.
Alawites
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 9th-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.
Despite that, many Alawites initially joined the uprising against Bashar al-Assad, calling for greater freedom and government transparency.
As the conflict progressed, however, Sunni rebels targeted Alawite communities, pushing them back into Assad's arms.
To give you some sense of how some Syrian Sunnis feel about Alawites, here's what Adnan Anour, a cleric who fled to Saudi Arabia, has said: "As for those Alawites who violate what is sacred, when the Muslims rule and are the majority of 85%, we will chop you up and feed you to the dogs."
Shiites
In May it appeared the rebels had the momentum and Assad's fall was just days away. Then Hezbollah, a Lebanese militant group, announced that it was joining the fray, and backing Assad.
Within weeks, this fierce group, led by Hassan Nasrallah, had managed to wrestle key cities from rebel control, turning the war's tide.
There aren't many Shiites in Syria, but the Assads courted them from neighboring Iran, Iraq and Lebanon, allowing them to build major shrines to the faith's founders in Syrian cities.
The strategy seems to have worked.
When Sunni rebels attacked those shrines, Shiites rushed in to defend them. Not that Sunnis and Shiites need many excuses to fight. They've been battling since the earliest days of Islam and continue to clash in Iraq and other countries.
Nasrallah harkened back to those early clashes when Hezbollah entered the fray, calling the Syrian Sunni rebels "murderers of Hussein."
Hussein ibn Ali was the Prophet Muhammad's grandson who refused to pledge allegiance to the ruling Muslim caliph in the 7th century. Shiites believe that he and his family were the rightful rulers of the Muslim community.
Sunnis
Sunni Muslims are by far the biggest Muslim sect, in the world and in Syria. It's estimated that Sunnis make up 75% of Syria's population of 22 million. But they've long been sidelined by the Assads.
It's little surprise, then, that most of the Free Syrian Army, the largest rebel group, is Sunni.
Within the Sunni coalition, there are remnants of the Muslim Brotherhood, who were brutally suppressed by the Assads; Salafists, who believe in a purified Islam based on its earliest days; and more secular-minded Sunnis.
In recent months they've been joined - sometimes to their consternation - by fighters from al Qaeda-linked groups. Always eager to fight Shiites and sow discord, these jihadists are every bit as fierce and battle-tested as Hezbollah, their sworn enemy.
It's unclear, however, how al Qaeda itself is involved in Syria.
The Iraqi-branch commander reportedly overstepped his authority in June by announcing a merger with Syria's al-Nusra Front, earning a smackdown from Ayman al-Zawahiri, al-Qaeda's global leader.
At the same time, some Syrian fighters say they pretend to be al-Qaeda just to annoy the Assad regime.
Still, prominent Sunni Muslim cleric Sheikh Yusuf Qaradawi has called on all Sunnis to join the fight against the Shiites and Hezbollah, calling them Hizb al-Shaytan, the “Party of the Devil”
Saudi Arabia and Qatar are backing that call with their wallets, according to international reports, hoping to prevent Shiites from gaining a stronghold in the region.
Christians
Christians, who form about 10 percent of the Syrian population, are essentially middle men in this civil war, caught between Assad's army and the Sunni rebels.
Under Assad, Christians had more rights than in many Middle Eastern countries, with the freedom to worship and run schools and churches. Their rights were limited however. The Syrian constitution says the president must be Muslim, for example.
According to UN reports, rebel fighters have targeted Christian communities, shooting up factories and detonating car bombs in Christian neighborhoods.
In addition, many Christians - in Syria and in the United States - fear the fate of Christians should Sunni fundamentalists take power in Syria.
They, like the Alawites, have been pushed back into Assad's arms.
Republican Sen. Rand Paul of Kentucky, perhaps with an eye towards a presidential run in 2016, is among the latest to express concern for Syria's Christians.
"I think the Islamic rebels winning is a bad idea for the Christians," Paul said on NBC's "Meet the Press," on Sunday. "All of a sudden we'll have another Islamic state where Christians are persecuted."