Syria’s two-year civil war has claimed more than 100,000 lives and sent at least 1.7 million refugees spilling across its borders.
It has already affected neighboring countries, and now threatens to drag in the United States.
What began as a popular movement, demanding the removal of autocratic President Bashar Assad, has become an increasingly sectarian conflict.
About three-quarters of Syria's Muslims are from the Sunni sect. However, the Assad dynasty, which has ruled the country since 1970, is from the minority Alawite sect which is close to Shiite Islam.
The sectarian divide is important because it lies at the root of instability not only within Syria but also in Iraq and other pockets of the Middle East.
The Islamic Republic of Iran - a Shiite Muslim regional power - is a staunch ally of the Assad government and has been providing arms, military training and cash. If Assad falls, Iran will be deeply wounded.
Defense think tank Royal United Services Institute (RUSI) sees the Syria conflict as part of a regional power struggle.
“It is increasingly clear that the world is confronting a crisis that extends far beyond Syria, threatening to deteriorate into a regional conflict,” a recent briefing paper outlined. “Now part civil, part proxy, it has also become a great power struggle between Saudi Arabia and Iran.”
After American troops toppled Sunni Saddam Hussein in 2003, Iraq’s Shiite majority became dominant. With this shift came regional alliances with Shiite countries including Iran and Syria.
Iraq provides a helping hand to the Assad regime by allowing Iranian arms and fighters to cross into Syria from Iraq, according to Secretary of State John Kerry. It also allows vital resupply flights from Iran to cross its airspace.
But this support is not without its costs: Violence is slipping across the border into Iraq and inflaming existing tensions. Deadly attacks are growing in Iraq, with the number of deaths due to violence back up at 2008 levels.
RUSI warns that if the Syria conflict continues, “the most important casualty of war is potentially Iraq.”
Russia is the Syrian government’s most powerful foreign backer and has stuck its neck out to support Assad’s crackdown.
It is helping the regime with weapons and even supports the growing involvement of Lebanese militia, Hezbollah. It is also reportedly helping Syria’s elite get around international financial sanctions, and has been protecting the regime from U.N. sanctions by using its veto power in the Security Council.
One theory for why Russia has been so resolute in its backing of Damascus is to ensure continued control of its only Mediterranean naval base, which is in the Syrian port of Tartus.
Russia has also invoked the specter of Islamic extremism in Syria, saying that Alawite Assad is fighting Sunni Muslim radicals.
The kingdom is the Syria opposition’s main backer, along with smaller Gulf state of Qatar, and has been sending billions in humanitarian aid and weapons to Syrian rebels.
It is providing advanced weaponry, including anti-aircraft and anti-tank missiles that it hopes will tip the balance for the rebels.
It is a follower of the strict Wahabi branch of Sunni Islam. In backing anti-Assad forces, Saudi Arabia is challenging its main regional foe, Shiite Iran. Some experts believe the Sunni-Shiite divisions are being exaggerated for political reasons.
“Saudi Arabia and Qatar have been willing to use sectarian language to make the Syrian conflict seem like a widespread attack on the Sunni population,” according to Chris Phillips, a lecturer at Queen Mary, University of London. “It’s part of a regional cold war with Iran on the battlefield of Syria.”
The small kingdom may exist in the shadow of neighbor Saudi Arabia, but it has big money and ambitions. It has been backing the rebels with humanitarian aid as well as arms, and is has been seeking a more prominent role in the region, offering to host peace talks in its capital, Doha.
Like Saudi Arabia, it is a follower of the strict Wahabi branch of Sunni Islam.
While the American government has supported anti-Assad forces with non-lethal assistance such as training, body armor, communications equipment and food aid, it has deep misgivings over whether to become more involved in the conflict.
Still smarting from unpopular interventions in Afghanistan and Iraq, the U.S. faces fears that deeper engagement in Syria will embroil it in yet another hard-to-exit regional conflict. There are also widespread concerns that American weapons will fall into the hands of radical al Qaeda-linked groups fighting with the rebels.
“Once we take action, we should be prepared for what comes next,” Chairman of the Joint Chiefs of Staff Army Gen. Martin Dempsey said in a letter to Congress. “Deeper involvement is hard to avoid.”
Until June, the administration was opposed to providing any lethal assistance to Syria's rebels, but in June it said it was moving ahead with sending weapons to vetted members of the opposition.
Formerly a close friend of Syria, Turkey’s moderate Islamist government has become a supporter of the rebels and supplying them with arms, security sources and diplomats say, according to Reuters. Officials also look the other way as rebels use the long and porous border as a resupply route.
At Turkey’s request, NATO has installed Patriot missile defenses on the border to protect it from spillover from the war in Syria. The move was aimed at calming Turkey's fears that it could come under attack from Syria.
Turkey also houses 500,000 Syrians in towns and relatively well-equipped refugee camps on the long border with Syria. There is growing anger in Turkey against the newcomers, though, especially after a bombing in the border town of Reyhanli killed more than 50 people.
France, the former colonial power in Syria, was the first Western power to join the anti-Assad camp and has been pushing for a more committed international effort to help the rebels.
It is increasingly concerned that Syrian rebels are losing ground and has suggested it will boost shipments of technical, medical and humanitarian aid to the rebels, but declined request for weapons and ammunition.
CAUGHT IN THE MIDDLE
Schisms in Syria are mirrored in its tiny neighbor Lebanon, which is a mosaic of religious communities, each with their own allegiances.
Christian and Sunni Muslim communities strongly oppose Assad’s regime, remembering the oppression of the 30-year Syrian occupation of Lebanon.
On the other side is Hezbollah. Of all the regional players, the powerful Shiite militia that controls much of Lebanon - and forms part of the government - may have the most at stake in the civil war. Should Assad fall, the military wing of Hezbollah will find it very hard to resupply arms.
It is for this reason that Hezbollah leader Hassan Nasrallah has decided to publicly throw his weight behind the Assad regime. Hezbollah fighters have great experience in guerilla warfare thanks to more than 30 years of confrontation with Israel.
Hezbollah's entry into the Syrian civil war has worsened the already tense relationships between Lebanon’s various religious communities, making it more likely that it will slip back into civil war itself.
The estimated 540,000 Syrian refugees in Jordan are a huge strain on the ailing economy. Meanwhile, King Abdullah’s government is struggling not to get too embroiled in the Syrian civil war, and stresses that that it wants a political solution.
Nevertheless, Jordan allows American troops to train Syrian rebels on its territory and F-16 jets are positioned there.
There are widespread fears in Jordan that Assad will deploy “sleeper” terrorist cells to destabilize the country if it openly backs the rebels. A Jordanian official recently told NBC News that officials there believe such an attack is inevitable.
Hamas: The Palestinians
The militant Sunni Palestinian group that governs Gaza used to have an important base in Syria’s capital Damascus, where it was both hosted by, and held hostage to, the whims of the Assad regime.
Hamas withdrew in 2011 after receiving support from Islamist movements sparked by the Arab Spring, and is now supporting Assad’s enemies. Palestinians in Syria are paying the price for the betrayal of their former host and patron, and have been driven out or targeted by bombings. Over 235,000 Palestinian refugees in Syria have been displaced, and more than 80,000 are now refugees again in neighboring countries, according to the U.N.
Although Syria and Israel are officially still at war, the relationship between the two has been relatively stable.
One of Israel’s main worries is that Islamist militants will enter Israel through the Golan Heights, which Israel captured from Syria in the 1967 Middle East war. While worrying that the Golan could become a springboard for attacks by militants among the rebels battling Assad, Israel has said repeatedly it does not want to be drawn into the fighting.
"We are not seeking to challenge anyone, but no one will harm the State of Israel without a response -- a strong and resolute response," Prime Minister Benjamin Netanyahu told soldiers who took part in a Golani infantry brigade exercise in June.
Israel is believed to have carried out three bomb strikes this year in Syria targeting Hezbollah weapons caches but refuses to confirm or deny whether it was responsible.
Although the civil war is centered on Syria, it is risks inflaming unrest across the Middle East. “The whole region could face a sustained period of violence,” University of London's Chris Phillips added.
NBC News' F. Brinley Bruton and Ghazi Balkiz contributed to this report.