A peaceful uprising against the president of Syria 10 years ago turned into a full-scale civil war. The conflict has left more than 380,000 people dead, devastated cities and drawn in other countries.
How did the Syrian war start?
Even before the conflict began, many Syrians were complaining about high unemployment, corruption and a lack of political freedom under President Bashar al-Assad, who succeeded his father, Hafez, after he died in 2000.
In March 2011, pro-democracy demonstrations erupted in the southern city of Deraa, inspired by uprisings in neighbouring countries against repressive rulers.
When the Syrian government used deadly force to crush the dissent, protests demanding the president's resignation erupted nationwide.
The unrest spread and the crackdown intensified. Opposition supporters took up arms, first to defend themselves and later to rid their areas of security forces. Mr Assad vowed to crush what he called "foreign-backed terrorism".
The violence rapidly escalated and the country descended into civil war. Hundreds of rebel groups sprung up and it did not take long for the conflict to become more than a battle between Syrians for or against Mr Assad. Foreign powers began to take sides, sending money, weaponry and fighters, and as the chaos worsened extremist jihadist organisations with their own aims, such as the Islamic State (IS) group and al-Qaeda, became involved. That deepened concern among the international community who saw them as a major threat.
Syria's Kurds, who want the right of self-government but have not fought Mr Assad's forces, have added another dimension to the conflict.
How many people have died?
The Syrian Observatory for Human Rights (SOHR), a UK-based monitoring group with a network of sources on the ground, had documented the deaths of 387,118 people by December 2020, among them 116,911 civilians.
The toll did not include 205,300 people who it said were missing and presumed dead, including 88,000 civilians believed to have died of torture in government-run prisons.
Another monitoring group, the Violations Documentation Center, which relies on information from activists across the country, has recorded what it considers violations of international humanitarian law and human rights law, including attacks on civilians.
It had documented 226,374 battle-related deaths, including 135,634 civilians, as of December 2020.
Almost 12,000 children have been killed or wounded, according to the UN children's agency Unicef.
The government's key supporters have been Russia and Iran, while Turkey, Western powers and several Gulf Arab states have backed the opposition to varying degrees over the past decade.
Russia - which had military bases in Syria before the war - launched an air campaign in support of Mr Assad in 2015 that has been crucial in turning the tide of the war in the government's favour.
The Russian military says its strikes only target "terrorists" but activists say they regularly kill mainstream rebels and civilians.
Iran is believed to have deployed hundreds of troops and spent billions of dollars to help Mr Assad.
Thousands of Shia Muslim militiamen armed, trained and financed by Iran - mostly from Lebanon's Hezbollah movement, but also Iraq, Afghanistan and Yemen - have also fought alongside the Syrian army.
The US, UK and France initially provided support for what they considered "moderate" rebel groups. But they have prioritised non-lethal assistance since jihadists became the dominant force in the armed opposition.
A US-led global coalition has also carried out air strikes and deployed special forces in Syria since 2014 to help an alliance of Kurdish and Arab militias called the Syrian Democratic Forces (SDF) capture territory once held by IS militants in the north-east.
Turkey is a major supporter of the opposition, but its focus has been on using rebel factions to contain the Kurdish YPG militia that dominates the SDF, accusing it of being an extension of a banned Kurdish rebel group in Turkey.
Turkish troops and allied rebels have seized stretches of territory along Syria's northern border and intervened to stop an all-out assault by government forces on the last opposition stronghold of Idlib.
Saudi Arabia, which is keen to counter Iranian influence, armed and financed the rebels at the start of the war, as did the kingdom's Gulf rival, Qatar.
Israel, meanwhile, has been so concerned by what it calls Iran's "military entrenchment" in Syria and shipments of Iranian weapons to Hezbollah and other Shia militias that it has conducted air strikes with increasing frequency in an attempt to thwart them.
How has the country been affected?
As well as causing hundreds of thousands of deaths, more than 2.1 million civilians have suffered injuries or permanent disabilities as a result of the conflict, according to the SOHR.
More than half of Syria's pre-war population of 22 million have fled their homes. Some 6.7 million are internally displaced, many of them living in camps, while another 5.6 million are registered as refugees abroad. Neighbouring Lebanon, Jordan and Turkey, which are hosting 93% of them, have struggled to cope with one of the largest refugee exoduses in recent history. One million Syrian refugee children have been born in exile.
As of January 2021, 13.4 million people inside Syria were in need of some form of humanitarian assistance, including 6 million in acute need, according to the UN. More than 12 million were struggling to find enough food each day and half a million children were chronically malnourished.
In the past year the humanitarian crisis has been compounded by an unprecedented economic downturn, which saw the value of Syria's currency decline drastically and food prices reach historic highs. In addition, the country has suffered a Covid-19 outbreak, whose true extent is not known because of limited testing capacity and a devastated healthcare system.
Entire neighbourhoods and vital infrastructure across the country also remain in ruins after a decade of fighting. UN satellite analysis suggested that more than 35,000 structures were damaged or destroyed in Aleppo city alone before it was recaptured by the government in late 2016.
And despite their protected status, 595 attacks on 350 separate medical facilities had been documented by Physicians for Human Rights as of March 2020, resulting in the deaths of 923 medical personnel. Such attacks have left only half of the country's hospitals fully functional.
Much of Syria's rich cultural heritage has also been destroyed. All six of the country's Unesco World Heritage sites have been damaged significantly, with IS militants deliberately blowing up parts of the ancient city of Palmyra.
UN war crimes investigators have accused all parties of perpetrating "the most heinous violations". "Syrians," their latest report says, "have suffered vast aerial bombardments of densely populated areas; they have endured chemical weapons attacks and modern day sieges in which perpetrators deliberately starved the population along medieval scripts and indefensible and shameful restrictions on humanitarian aid".
Who is in control of the country now?
The government has regained control of Syria's biggest cities, but large parts of the country are still held by rebels, jihadists and the Kurdish-led SDF.
The last remaining opposition stronghold is in the north-western province of Idlib and adjoining parts of northern Hama and western Aleppo provinces.
The region is dominated by a jihadist alliance called Hayat Tahrir al-Sham (HTS), but is also home to mainstream rebel factions. An estimated 2.7 million displaced people, including a million children, are living there, many of them in dire conditions in camps.
In March 2020, Russia and Turkey brokered a ceasefire to halt a push by the government to retake Idlib. There has been a relative calm since then, but it could break down at any moment.
In the country's north-east, Turkish forces and allied Syrian rebels launched an offensive against the SDF in October 2019 to create a "safe zone" clear of Kurdish YPG militia along the Syrian side of the border, and have occupied a 120km (75 miles) long stretch since.
To halt the assault the SDF struck a deal with the Syrian government that saw the Syrian army return to the Kurdish-administered region for the first time in seven years. The government has vowed to eventually regain full control of it.
Will the war ever end?
It does not look like it will anytime soon, but everyone agrees a political solution is required.
The UN Security Council has called for the implementation of the 2012 Geneva Communiqué, which envisages a transitional governing body "formed on the basis of mutual consent".
Nine rounds of UN-mediated peace talks - known as the Geneva II process - failed to make progress, with President Assad apparently unwilling to negotiate with political opposition groups that insist he must step down as part of any settlement.
Russia, Iran and Turkey set up parallel political talks known as the Astana process in 2017.
An agreement was reached the following year to form a 150-member committee to write a new constitution, leading to free and fair elections supervised by the UN. But in January 2021, UN special envoy Geir Pedersen lamented that they had not even begun drafting any reforms.
Mr Pedersen also noted that, with five foreign armies active in Syria, the international community could not pretend the solutions to the conflict were only in the hands of the Syrians.