Why is there a war in Syria?
A peaceful uprising against the president of Syria seven years ago has turned into a full-scale civil war. The conflict has left more than 350,000 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 late father Hafez in 2000.
In March 2011, pro-democracy demonstrations erupted in the southern city of Deraa, inspired by the "Arab Spring" in neighbouring countries.
When the 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.
How many people have died?
The Syrian Observatory for Human Rights, a UK-based monitoring group with a network of sources on the ground, had documented the deaths of 353,900 people by March 2018, including 106,000 civilians.
The figure did not include 56,900 people who it said were missing and presumed dead. The group also estimated 100,000 deaths had not been documented.
Meanwhile, the Violations Documentation Center, which relies on activists inside Syria, has recorded what it considers violations of international humanitarian law and human rights law, including attacks on civilians.
It had documented 185,980 battle-related deaths, including 119,200 civilians, by February 2018.
What is the war about?
It is now more than a battle between those for or against Mr Assad.
Many groups and countries - each with their own agendas - are involved, making the situation far more complex and prolonging the fighting.
They have been accused of fostering hatred between Syria's religious groups, pitching the Sunni Muslim majority against the president's Shia Alawite sect.
Such divisions have led both sides to commit atrocities, torn communities apart and dimmed hopes of peace.
They have also allowed the jihadist groups Islamic State (IS) and al-Qaeda to flourish.
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.
Who's involved?
The government's key supporters are Russia and Iran, while the US, Turkey and Saudi Arabia back the rebels.
Russia - which already had military bases in Syria - 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, France and other Western countries have provided varying degrees of support for what they consider "moderate" rebels.
A global coalition they lead has also carried out air strikes on IS militants in Syria since 2014 and helped an alliance of Kurdish and Arab militias called the Syrian Democratic Forces (SDF) capture territory from the jihadists.
Turkey has long supported the rebels but it has focused on using them to contain the Kurdish militia that dominates the SDF, accusing it of being an extension of a banned Kurdish rebel group in Turkey.
Saudi Arabia, which is keen to counter Iranian influence, has also armed and financed the rebels.
Israel, meanwhile, has been so concerned by shipments of Iranian weapons to Hezbollah in Syria that it has conducted air strikes in an attempt to thwart them.
How has the country been affected?
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As well as causing hundreds of thousands of deaths, the war has left 1.5 million people with permanent disabilities, including 86,000 who have lost limbs.
At least 6.1 million Syrians are internally displaced, while another 5.6 million have fled abroad.
Neighbouring Lebanon, Jordan and Turkey, where 92% of them now live, have struggled to cope with one of the largest refugee exoduses in recent history.
The UN estimates 13.1 million people will require some form of humanitarian help in Syria in 2018.
The warring parties have made the problems worse by refusing aid agencies access to many of those in need. Almost 3 million people live in besieged or hard-to-reach areas.
Syrians also have limited access to healthcare.
Physicians for Human Rights had documented 492 attacks on 330 medical facilities by the end of December 2017, resulting in the deaths of 847 medical personnel.
Much of Syria's rich cultural heritage has also been destroyed. All six of the country's six Unesco World Heritage sites have been damaged significantly.
Entire neighbourhoods have been levelled across the country.
A recent UN assessment found 93% of buildings had been damaged or destroyed in one district of the rebel-held Eastern Ghouta region near Damascus.
How is the country divided?
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The government has regained control of Syria's biggest cities but large parts of the country are still held by rebel groups and the Kurdish-led SDF alliance.
The largest opposition stronghold is the north-western province of Idlib, home to more than 2.6 million people.
Despite being designated a "de-escalation zone", Idlib is the target of an offensive by the government, which says it is targeting jihadists linked to al-Qaeda.
A ground assault is also under way in the Eastern Ghouta. Its 393,000 residents have been under siege by the government since 2013, and are facing intense bombardment as well as severe shortages of food and medical supplies.
The SDF meanwhile controls most territory east of the River Euphrates, including the city of Raqqa. Until 2017, it was the de facto capital of the "caliphate" proclaimed by IS, which now controls only a few pockets across Syria.
Will the war ever end?
It does not look like it will any time soon but everyone agrees a political solution is required.
The UN Security Council has called for the implementation of the 2012 Geneva Communique, which envisages a transitional governing body "formed on the basis of mutual consent."
But nine rounds of UN-mediated peace talks - known as the Geneva II process - since 2014 have shown little progress.
President Assad has appeared increasingly unwilling to negotiate with the opposition. The rebels still insist he must step down as part of any settlement.
Meanwhile, Western powers have accused Russia of undermining the peace talks by setting up a parallel political process.
The so-called Astana process saw Russia host a "Congress of National Dialogue" in January 2018. However, most opposition representatives refused to attend.
