Big questions loom over the debate on whether the United States should strike Syria: Who are the Syrian rebels? Should the United States and other countries help them?
The first thing to know is that the rebels aren't all playing on the same team. They're arrayed against the Syrian government in a constellation of groups and factions, each with its own agenda. Some are in league with al-Qaida.
Secondly, the opposition has morphed in the last few years. It started with ordinary Syrians angry at police for arresting children who painted anti-government graffiti. Now it attracts fighters from outside Syria.
What else should you know about the rebels in Syria? A lot.
Here are 20 points to get you up to speed, based on CNN's reporting since the Syrian crisis began in 2011.
1. The opposition didn't start out as a military movement.
Peaceful protests against President Bashar al- Assad's government are how it all started in February 2011, after authorities arrested 15 schoolchildren for painting anti-government graffiti on the walls of a school in the city of Daraa.
Syrian security forces opened fire at one demonstration, killing at least four protesters -- the first casualties, activists say, in Syria's civil war.
2. But it wasn't long before things grew more violent.
As anti-government protests spread across Syria that year, calls for reforms quickly escalated into calls for the removal of the entire al-Assad regime.
In July 2011, seven Syrian military officers appeared in a YouTube video announcing their defection, calling themselves the "Free Syrian Army" and promising to wage guerrilla war against al-Assad.
3. Some rebel groups are closely allied with al-Qaida.
Syria's al-Qaida wing is known as the Islamic State of Iraq and Syria, and it's been gaining a greater foothold.
And analysts say al-Qaida's affiliate in Syria, Jabhat al-Nusra, is generally the most effective force fighting al-Assad. The group's name means "Victory Front." It was listed as a foreign terrorist organization by the U.S. State Department in December.
4. That's one reason why many in the West have expressed qualms about helping them.
It's been a key concern expressed in debates on Capitol Hill this week.
The Syria rebels have promised U.S. and European officials that any military weaponry they get won't end up in extremists' hands. But that hasn't quelled criticism from some quarters that helping the rebels is a dangerous risk.
Russian President Vladimir Putin has cited extremism among the rebels as he slammed decisions by the West to arm the opposition. At an economic forum in June, he cautioned, "Where will those weapons end up?"
5. But not all the rebels are jihadists. Look at the Free Syrian Army, for example.
The Free Syrian Army's ranks swelled with soldiers who said they'd rather defect from the government military than obey orders to fire on protesters. But there also civilians among them.
The group first emerged in July 2011, claiming responsibility for an attack on an air intelligence base.
6. There are also a lot of local militias.
Imagine the Minutemen during the American Revolutionary War, says Andrew Tabler, a senior fellow at The Washington Institute for Near East Policy.
"It's extremely complicated to deal with them because there are so may voices among the opposition," Tabler says.
The rebel militias are composed in large part of defector soldiers. But there are also many civilians, including students, shopkeepers, real-estate agents, and even members of al-Assad's ruling Ba'ath party.
7. There have been efforts to unite the rebels, but still there's no central figure leading them. And if the regime falls, it's anybody's guess what could happen next.
An organization known as the Supreme Military Council, which formed late last year, now unites many rebel groups.
And for now, the rebels are working together to achieve a common goal -- toppling al-Assad's government. Here's how one local al-Nusra front leader put it to CNN in April: "In the period after the regime falls, our main goal is to create an Islamic state that is ruled by the Koran. It can have civilian institutions, but not democracy. We look at the other Free Syrian Army rebels as one of many groups defending religion, so we support them. In the future, we will handle this differently."
8. Religion motivates many of them.
The rebels are largely made up of Sunni Muslims battling against al-Assad's minority Alawite sect, which is associated with Shia Islam. Weapons and funds from Iran's Shia rulers have helped the Syrian regime, while Sunni states like Saudi Arabia have reportedly supported Syrian rebels.
"The conflict has become increasingly sectarian, with the conduct of the parties becoming significantly more radicalized and militarized," the UN said earlier this year.
That doesn't bode well for Syria's future. Studies have said religious civil wars are longer and bloodier than other types of clashes. They're also twice as likely to recur and twice as deadly to noncombatants.
9. They're not all Syrian.
Thousands of foreign fighters are believed to have traveled to Syria to join the rebels since early 2011. The Washington Institute for Near East Policy estimates the total at between 2,000 and 5,500.
That group includes hundreds of Europeans, the institute says. And there have also been reports of several people from the United States fighting with the rebels.
10. They're not just fighting al-Assad's regime -- they're fighting Hezbollah militants.
Early on in the Syrian conflict, reports surfaced that Hezbollah fighters were helping Syrian government forces. In May, the Lebanon-based Shiite militant group's leader confirmed it.
"Syria is the backbone of the resistance (in the region) and its main supporter," Hezbollah Secretary-General Hassan Nasrallah said in a televised speech. "The resistance will never stand by while its backbone is exposed."
11. It's not all about fighting on the battlefield.
The Syrian National Coalition is an umbrella group representing the Syrian opposition that formed last year.