“It was like hell. In the hospital it was very quiet despite all the people. Everyone was crying, crying from the gas, crying from looking at the bodies. It was so full, people were laid out on the floor. I almost stepped on the body of my friend. I didn’t even realise it was him,” said Nour Aden, an activist from East Ghouta, remembering the events of 21 August 2013.

Four years ago, thousands of people in the besieged rebel district of Damascus were rushed to hospital – after an air raid in the early hours – with symptoms such as convulsions, suffocation, coughing up blood and foaming at the mouth.

In the then two years since Syria’s civil war broke out, doctors had grown used to treating trauma and conflict wounds. But the overwhelmed medical staff didn’t know how to treat these patients with no visible signs of injury. Children dropped like flies in front of them because of what international investigators would later confirm were the effects of sarin gas, a chemical agent that targets the central nervous system.

‘It’s a different kind of fear in Ghouta now ... You hear the planes come and you fear the bombing. But the chemicals are silent. You don’t know you’re dying until you can smell it, and then it is too late’ (AFP/Getty)

It’s still not known how many people died – estimates range from 281 to 1,729. All sides agree, however, that East Ghouta was one of the worst chemical incidents in modern history.

Images of entire families dead in their beds, with dark rings around their mouths and eyes and faces contorted in pain, caused outrage around the world. The Syrian regime had crossed a “red line”, the then US President Barack Obama had said. Military intervention was proposed in a bill that never actually made it to a floor vote in the House or Senate.

“Usually, when a bad thing happens, with time the wounds heal and the bad memories fade,” said Dr Kassam Eid, who treated patients during the attack.

“Unlike the other bad things in my life, and there have been many, the more time passes without any accountability or seeing these attacks stop, the more and more painful it gets. Every new chemical attack against civilians reminds me we are not treated like humans by the wider world or the regime.”

While the international watchdog Organisation for the Prevention of Chemical Weapons (OPCW) did not explicitly blame Syrian President Bashar al-Assad’s government, there is no other fighting force in Syria capable of launching such a huge attack.

Syrian, Russian and far-right and far-left conspiracy claims that it was a “false flag” attack by the rebels have been thoroughly debunked – but the regime agreed to give up its chemical weapons stocks in the wake of the Ghouta incident as a mark of transparency. Assad has repeatedly denied using chemical weapons, and continues to blame rebels for attacks.

Michael Fallon says UK will support further action in Syria to stop chemical attacks

Since then, however, war monitors such as the UK-based Syrian Observatory for Human Rights report there have been dozens of alleged chlorine attacks and at least one major sarin attack.

Officials from the Obama administration said they always believed it was possible some weapons had been held back, trying to refer instead to the destruction of Syria’s “declared” chemical weapons stocks, although the nuance has often been lost.

While the OPCW team in Syria has carried out 18 chemical site visits since 2013, it has effectively given up, Reuters reported last week, because Syria has failed to provide sufficient or accurate information as to the operation of its facilities.

In April this year, a sarin attack in the rebel-held village of Khan Sheikhoun, which killed around 130 civilians, once again led to calls for Assad to be held accountable for gassing his own people.

This time, US President Donald Trump was quick to retaliate.

Mr Trump ordered what the White House called a “warning shot” barrage of 59 Tomahawk missiles which struck the regime-operated Shayrat airbase near Homs in what marked the first direct action against Assad’s forces taken by the US since the civil war began more than six years ago.

In pictures: US missile strike against Syria Show all 7 1 /7 In pictures: US missile strike against Syria In pictures: US missile strike against Syria The guided-missile destroyer USS Porter (DDG 78) launches a tomahawk land attack missile in the Mediterranean Sea AP In pictures: US missile strike against Syria The United States military launched at least 50 tomahawk cruise missiles at al-Shayrat military airfield near Homs, Syria, in response to the Syrian military's alleged use of chemical weapons in an airstrike in a rebel held area in Idlib province EPA In pictures: US missile strike against Syria Shayrat airfield in Syria Getty Images In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria President Donald Trump speaks at Mar-a-Lago in Palm Beach, Fla., after the US fired a barrage of cruise missiles into Syria in retaliation for this week's gruesome chemical weapons attack against civilians AP In pictures: US missile strike against Syria Syria's President Bashar al-Assad Reuters

The incident sparked fears that the US would become further entangled in Syria’s complex and multi-sided war. But for many Syrians, the military action was welcome.

“I was very happy about the strike. It was a message to Assad that he cannot use these weapons with impunity,” Dr Eid said. “But I don’t think the US really cares about what happens to the Syrian people any more, no matter what it says. It has abandoned the country to Russia.”

The opposition says that there have been eight small chemical attacks on rebel-held areas in Syria even since the Khan Sheikhoun incident.

Until the war ends, Mr Aden said, people would continue to live in fear. “It’s a different kind of fear in Ghouta now,” he said. “You hear the planes come and you fear the bombing. But the chemicals are silent. You don’t know you’re dying until you can smell it and then it is too late.

“The Assad regime cannot be trusted not to gas us again, and we will always be afraid.”

“All of them – the UN, the US, Russia, Iran – they all say they want the best for the Syrian people, but that is a lie,” Dr Eid added.