How did the war begin?
Long before the conflict began, many Syrians complained about high unemployment, widespread corruption, a lack of political freedom and state repression under President Bashar al-Assad, who succeeded his father, Hafez, in 2000.
In March 2011, pro-democracy demonstrations inspired by the Arab Spring erupted in the southern city of Deraa. The government's use of deadly force to crush the dissent soon triggered nationwide protests demanding the president's resignation.
As the unrest spread, the crackdown intensified. Opposition supporters began to take up arms, first to defend themselves and later to expel security forces from their local areas. Mr Assad vowed to crush "foreign-backed terrorism" and restore state control.
The violence rapidly escalated and the country descended into civil war as hundreds of rebel brigades were formed to battle government forces for control of the country.
Why has the war lasted so long?
In essence, it has become more than just a battle between those for or against Mr Assad.
A key factor has been the intervention of regional and world powers, including Iran, Russia, Saudi Arabia and the United States. Their military, financial and political support for the government and opposition has contributed directly to the intensification and continuation of the fighting, and turned Syria into a proxy battleground.
External powers have also been accused of fostering sectarianism in what was a broadly secular state, pitching the country's Sunni majority against the president's Shia Alawite sect. Such divisions have encouraged both sides to commit atrocities that have not only caused loss of life but also torn apart communities, hardened positions and dimmed hopes for a political settlement.
Jihadist groups have also seized on the divisions, and their rise has added a further dimension to the war. Hayat Tahrir al-Sham, an alliance formed by what was once the al-Qaeda-affiliated al-Nusra Front, controls large parts of the north-western province of Idlib.
Meanwhile, so-called Islamic State (IS), which controls large swathes of northern and eastern Syria, is battling government forces, rebel brigades and Kurdish militias, as well as facing air strikes by Russia and a US-led multinational coalition.
Thousands of Shia militiamen from Iran, Lebanon, Iraq, Afghanistan and Yemen say they are fighting alongside the Syrian army to protect holy sites.
Why are so many outside powers involved?
Russia, for whom President Assad's survival is critical to maintaining its interests in Syria, launched an air campaign in September 2015 with the aim of "stabilising" the government after a series of defeats. Moscow stressed that it would target only "terrorists", but activists said its strikes mainly hit Western-backed rebel groups.
Six months later, having turned the tide of the war in his ally's favour, President Vladimir Putin ordered the "main part" of Russia's forces to withdraw, saying their mission had "on the whole" been accomplished. However, intense Russian air and missile strikes went on to play a major role in the government's siege of rebel-held eastern Aleppo, which fell in December 2016.
Shia power Iran is believed to be spending billions of dollars a year to bolster the Alawite-dominated government, providing military advisers and subsidised weapons, as well as lines of credit and oil transfers. It is also widely reported to have deployed hundreds of combat troops in Syria.
Mr Assad is Iran's closest Arab ally and Syria is the main transit point for Iranian weapons shipments to the Lebanese Shia Islamist movement Hezbollah, which has sent thousands of fighters to support government forces.
The US, which says President Assad is responsible for widespread atrocities, has provided only limited military assistance to "moderate" rebel groups, fearful that advanced weapons might end up in the hands of jihadists. The US has conducted air strikes on IS in Syria since September 2014, and, in the first intentional attack on Syria itself, hit an air base which it said was behind a deadly chemical attack, in April 2017.
Sunni-ruled Saudi Arabia, which is seeking to counter the influence of its rival Iran, has been a major provider of military and financial assistance to the rebels, including those with Islamist ideologies.
Turkey is another staunch supporter of the rebels. However, it has sought to contain the Kurdish Popular Protection Units (YPG) militia whose fighters are battling IS as part of the US-backed Syrian Democratic Forces (SDF) alliance. Ankara accuses the YPG of being an extension of the banned Turkish Kurdistan Workers' Party (PKK).
In August 2016, Turkish troops backed a rebel offensive to drive IS militants out of one of the last remaining stretches of the Syrian side of the border not controlled by the Kurds. Since then, they have taken control of some 2,000 sq km (772 sq miles) of territory, according to the Turkish military, and forced the US to deploy troops to the SDF-controlled town of Manbij to prevent clashes.
What impact has the war had?
The UN says at least 250,000 people have been killed in the past five years. However, the organisation stopped updating its figures in August 2015. The Syrian Observatory for Human Rights, a UK-based monitoring group, puts the death toll at more than 321,000, while a think-tank estimated in February 2016 that the conflict had caused 470,000 deaths, either directly or indirectly.
Five million people - most of them women and children - have fled Syria, according to the UN. Neighbouring Lebanon, Jordan and Turkey have struggled to cope with one of the largest refugee exoduses in recent history.
About 10% of Syrian refugees have sought safety in Europe, sowing political divisions as countries argue over sharing the burden. A further 6.3 million people are internally displaced inside Syria.
The UN estimates it will need $3.4bn (£2.7bn) to help the 13.5 million people who will require some form of humanitarian assistance inside Syria in 2017.
Almost 85% of Syrians live in poverty, with more than two-thirds of the population in either extreme or abject poverty. More than 12.8 million people in Syria require health assistance and more than seven million are food insecure amid rising prices and food shortages. Households spend up to a quarter of their income just on water. Some 1.75 million children are out of school.
The warring parties have compounded the problems by refusing humanitarian agencies access to many of those in need. Some 4.9 million people live in besieged or hard-to-reach areas.
What's being done to end the conflict?
With neither side able to inflict a decisive defeat on the other, the international community long ago concluded that only a political solution could end the conflict. The UN Security Council has called for the implementation of the 2012 Geneva Communique, which envisages a transitional governing body with full executive powers "formed on the basis of mutual consent".
Peace talks in early 2014, known as Geneva II, broke down after only two rounds, with the UN blaming the Syrian government's refusal to discuss opposition demands.
A year later, the conflict with IS lent fresh impetus to the search for a political solution in Syria. The US and Russia persuaded representatives of the warring parties to attend "proximity talks" in Geneva in January 2016 to discuss a Security Council-endorsed road map for peace, including a ceasefire and a transitional period ending with elections.
The first round broke down while still in the "preparatory" phase, as government forces launched an offensive around Aleppo. The talks resumed in March 2016, after the US and Russia brokered a nationwide "cessation of hostilities" that excluded jihadist groups. But they collapsed the following month.
Turkey and Russia brokered another truce after the fall of Aleppo. In January 2017, they and Kazakhstan hosted the first face-to-face meeting between rebel fighters and government officials since the war began. That was followed by a fresh round of UN-mediated talks in Geneva, which UN envoy Staffan de Mistura said "achieved much more than many people had imagined we could have".
What is left of rebel territory?
The fall of Aleppo means the government now controls Syria's four biggest cities. But large parts of the country are still held by other armed groups.
Rebel fighters and allied jihadists are estimated to control about 15% of Syrian territory, according to the Syrian Observatory for Human Rights.
US officials said in early December 2016 that there were 50,000 or more "moderate" rebels, concentrated in the north-western province of Idlib and the western Aleppo countryside.
Rebels also control smaller areas in the central province of Homs, the southern provinces of Deraa and Quneitra, and the eastern Ghouta agricultural belt outside Damascus.
Kurdish forces, who say they support neither the government nor the opposition, meanwhile control much of Syria's border with Turkey, as well as a large part of the country's north-east.
And although they have suffered extensive losses in the past two years, IS militants still hold large parts of central and northern Syria, including the city of Raqqa.