Ten days ago my phone pinged. It was my plastic surgeon colleague from M1 Hospital in Aleppo asking my advice about a patient with a deep defect in his pelvis. He told me that around 20 patients were being admitted following a suspected chemical weapon attack with chlorine. His emergency department was full. He had not enough masks or oxygen cylinders available to treat the majority. He was not able to ventilate any of the seriously ill patients because of lack of equipment. Two patients subsequently died from the chlorine, which turned into hydrochloric acid and dissolved their lungs. He was devastated.

We continued the dialogue the following day when he told me that, from 8 o’clock in the morning until midnight, there was a constant barrage of bombs dropped by the Russian and Syrian air forces and there was a constant flood of dead and dying patients.

“Each day would be a mass casualty event,” says British surgeon David Nott, who has worked in Aleppo since 2013. Photograph: MSF/PA

A week later another colleague from M10 hospital rang me and told me he had treated Omran, the young boy whose picture had gone viral on social media. To be honest, I had not seen the picture. He simply wanted to tell me that the boy had survived and he had treated his deep scalp wound and he was going to be discharged home.

I asked him whether he wanted to go on television and contacted the BBC and Channel 4. He was willing to do this but not to show his face, as he would surely have been targeted by the regime. To see him talking on my television brought a lump to my throat. He is the kindest and nicest human being I’ve ever worked with. He has been there from the start of the war and his only words on the television were: “Please help us.”

I know exactly where he was and really wished I could be there with him. He has not done anything wrong: he is only there to help people, as are all my other colleagues and friends who are working round the clock. They are all exhausted and stressed. They will not leave because they have so much work to do.

The picture of this little boy must be a turning point in this war. It must not be looked at and forgotten within 24 hours. I have pictures on my phone and in my computer of all the cases that I dealt with when I was in Aleppo. These images are far worse than the image of Omran. They are of dead and dying children. I have given lectures showing these pictures and have had people in the audience crying. Why has the world become heartless?

When President Obama talked about crossing the red line after 400 children were killed by chemical weapons outside Damascus in 2013, he was awaiting the outcome of a vote on action in the British parliament. In the end it was close, with 285-272 voting against military action against the Syrian regime. If the west had shown strong leadership, I have no doubt that the Syrian military hierarchy would have collapsed. George Osborne said prophetically: “I hope this doesn’t become a moment when we turn our back on all the world’s problems.” It’s not often that a politician gets it right, but he did then.

I packed my gas mask and spent six weeks in Aleppo just after that statement in 2013. Aleppo was a very moderate city. The rebel side was controlled by the Free Syrian Army. It was promised help by the west and was expecting humanitarian aid and equipment to overthrow the Assad regime. Most of my time was spent teaching the surgeons how to perform very difficult trauma surgery following gunshot wounds and fragmentation wounds from Syrian jet airstrikes. On average we would treat about 12 to 14 gunshot wounds a day, and we had excellent backup support on the intensive care unit from intensive physicians in Washington manning the ventilators by Skype.

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I was protected by the Aleppo city medical council, who looked after me like a parent looks after a child. I was the only westerner in Aleppo then, as Isis had control of the north and made it very difficult to cross checkpoints. There were no reporters in Aleppo, so I came home and talked of the horrors I had seen as snipers from the Syrian regime played games with women and children’s lives.

When I returned to Aleppo in September 2014, Syrian jets were still flying low level around the city shooting at anybody using rockets, but worse were the Syrian helicopters dropping barrel bombs containing around 500kg of TNT indiscriminately on densely populated areas. Ambulances would bring in hordes of patients and each day would be a mass casualty event.

All people living in non-regime areas were classified as terrorists. I, as a humanitarian just going to operate and save lives, would have also been looked on as a terrorist and would have been jailed and tortured and most likely killed – as was Abbas Khan, my British orthopaedic colleague who volunteered in the same hospital as I did. October 2014 was a particularly awful period, not just for the residents of Aleppo but also for the western hostages, including journalists and humanitarians, who were facing being beheaded by Isis.

At the time I was probably about 30 miles away from where Alan Henning, who was delivering an ambulance to Aleppo, had the most terrible fate inflicted on him. It was just too dangerous for journalists to go in, and thus the horror was kept off the world’s radar. This allowed the Syrian regime to perpetuate the worst atrocities unhindered.

The intervention by Russia in September 2015 was made at the request of a beleaguered Assad regime. Assad’s forces have been falling apart ever since the conflict began and havehad to be propped up by militias from Iran and Hezbollah. It suited Russia to be involved. By standing up to the west he has become a global player and is challenging the monopoly of the influence in the Middle East which had been held by the US for decades.

The Syrian and Russian regimes did not appreciate the effect it would have on galvanising the opposition fighters. This has led to a significant increase in heavy Russian and Syrian airstrikes. This is always cyclical; the more the opposition gains, the heavier the response, and this is what we are seeing now. The picture of Omran epitomises the horror that can be broadcast on our television screens. It is significantly better than many of the pictures of children coming out from Aleppo but should still shame world leaders. Hopefully this will galvanise the west to finally stop sitting back and being passive observers. This war is different because it is our war. The refugees that have poured into Europe are there because we have not done anything to stop the suffering. If we wait for another five years, there will be one million people killed and 20 million refugees.

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The sticking point is whether Assad stays or goes. He has to go. The refugees who have left the country will not return unless he has gone. There is no alternative.

The British government has a new prime minister who is as steely as Sir John Major, who created a no-fly zone when the Kurds were being attacked by Saddam Hussein with chemical weapons. Theresa May must show some leadership. The first thing that must happen is to demand a no-fly zone for Syrian helicopters to stop them dropping their barrel bombs. The international community must get behind her and force this action.

David Nott is a Welsh general and vascular surgeon based in London, who has spent more than two decades working in some of the world’s most difficult and dangerous places. From Bosnia, Liberia, Afghanistan and Iraq to more recently a focus on Syria, he not only works as a doctor himself but also teaches advanced surgical skills to local medics and surgeons, and supports them remotely. In 2015 he established the David Nott Foundation with his wife Elly, which supports surgeons to develop their operating skills for war zones and austere environments.