Video footage of Syrian children choking to death in agony in a chemical attack on the north-western town of Khan Sheikhoun has alarmed the international community, triggering calls for tougher actions in the war-torn country.
The chemical assault on Tuesday (April 4), which has killed at least 70 people, many of them children and women, likely involved a banned nerve agent, the World Health Organisation (WHO) and medical experts say.
It's the deadliest chemical assault on a civilian population in Syria since a sarin nerve agent attack in 2013 by government forces on rebel-held suburbs of Damascus.
Here's a quick look at the conflict in Syria, now into its sixth year:
Why is there a civil war in Syria?
The trouble began in March 2011 in the city of Deraa where a peaceful uprising against President Bashar al-Assad took place. Locals protested after 15 schoolchildren were arrested and reportedly tortured for writing anti-government graffiti on a wall. The protest began peacefully, with the protesters asking for the release of the children and calling for democracy and greater freedom in the country.
The government, however, responded angrily and the army opened fire on the protesters on March 18, 2011, killing four people. The next day, they shot at mourners at the funerals, killing another person. The public outrage spread to other parts of the country, triggering nationwide protests demanding Mr Assad's resignation.
By July that year, hundreds of thousands of people were taking to the streets across Syria. Opposition supporters began taking up arms, first to defend themselves and later to push out security forces from their local areas.
The country eventually plunged into civil war, with anti-government rebel groups battling government forces for control of cities and towns. The fighting reached the Syrian capital Damascus and second city of Aleppo in 2012.
Successive rounds of peace talks, including a United Nations-sponsored meeting in Geneva in March 2017, have failed to produce a political breakthrough.
Who is fighting who?
Those demanding that Mr Assad steps down include rebel fighters, political parties who disagree with the Syrian leader, and those living in exile who cannot return to the country. The conflict has also pitched the country's Sunni majority against the president's Shia Alawite sect.
In addition, the Islamic State in Iraq and Syria (ISIS) has capitalised on the chaos and taken control of large swathes of Syria and Iraq, where it has proclaimed the creation of a caliphate in June 2014. Its foreign fighters are involved in a "war within a war" in Syria, battling rebels and rival militants from the Al-Qaeda-affiliated Nusra Front, as well as government and Kurdish forces.
How bad is the humanitarian crisis in Syria?
Almost half a million people are estimated to have been killed and more than 11 million, about half the population, are displaced either internally or as refugees, putting Syria in the international spotlight. Syrians fleeing the conflict are faced with rising anti-foreigner sentiment, or xenophobia, in countries that are grappling with security threats like terrorism and crimes.
The conflict has caused at least US$16 billion (S$23 billion) in damage to agriculture - about a third of the country's Gross Domestic Product, the UN Food and Agriculture Organisation (FAO) has said, calling for more funds to salvage local food production as millions go hungry. The fightings have led to extensive crop and livestock losses, and destroyed greenhouses, veterinary clinics, irrigation systems, tractors and other assets.
More than seven million people in Syria are classified as "food insecure", meaning they are not always sure where their next meal is coming from, according to the FAO.
Why and how does Russia support President Assad?
Russia has significant economic and military interests in Syria, such as a Mediterranean naval base at Tartus, and it is in Russia's interest that the Assad regime does not collapse.
President Vladimir Putin also wants to send a message to the world that Russia is still a force to be reckoned with. The US-led invasion of Iraq in 2003 and the 2011 toppling of Muammar Gadhafi's Libyan regime - helped by an international coalition bombing campaign - left Moscow deprived of key allies. Syria under Mr Assad is seen by the Kremlin as a key pillar of its strategic influence in the Middle East and Moscow is reluctant to let it go.
Moscow is also concerned with rising Islamic violence. It fears an ISIS victory in Syria would have reverberations at home, as some of the top military commanders of the militant group are Russian speakers of Chechen origin.
Because of its interests in Syria, Russia has blocked resolutions critical of Mr Assad at the UN Security Council and has continued to supply weapons to the Syrian military despite international criticism.
In response to the chemical attack on Khan Sheikhoun, France and the US have put forward a draft UN resolution demanding a full investigation into the attack. Russia's deputy ambassador Vladimir Safronkov told the UN council that the proposed resolution was hastily prepared and unnecessary, but he voiced support for an inquiry.
What's the US position on Syria?
Under Mr Barack Obama's administration, the US made Mr Assad's departure a key goal. In August 2013, a sarin gas attack around the town of Ghouta left hundreds dead and Washington said Mr Assad had crossed a "red line".
Mr Obama threatened an air campaign to topple him but called it off at the last minute when the Syrian leader agreed to give up his chemical arsenal under a deal brokered by Moscow.
In mid-2014 as Washington increased support to moderate rebels to fight Mr Assad's regime, US officials privately conceded that the Syrian leader wasn't going anywhere soon and admitted the difficulty in removing him. By September 2015, then US Secretary of State John Kerry said Mr Assad had to go but the timing of his departure should be decided through negotiation.
Before Mr Donald Trump became president, he had strongly urged Mr Obama not to order military intervention against the Assad regime. And he came to office promising both to improve ties with Syria's ally Russia and to focus US efforts in Syria solely on the defeat of the ISIS.
But following the chemical attack on Tuesday, Mr Trump condemned it as a "horrific" strike against "innocent people, including women, small children and even beautiful little babies".
Asked if it crossed a red line for him, he replied: "It crossed a lot of lines for me... Beyond a red line. Many, many lines."
Reports said his remarks may suggest a greater US role in protecting the Syrian population in a vicious civil war that he has always said the US should avoid.
SOURCES: BBC, Reuters, AFP