I was a teenager when I faced the worst of humanity. I remember our house burning to the ground and my family fleeing Srebrenica, hoping against hope for a chance to live. I remember the torture, and the smell of blood. I didn’t know it yet, but I was living through the worst genocide in Europe since the second world war. And afterwards, I remember the promises of “never again”.

Those promises are being broken, hour after hour, day after day, in the deepening horror of east Aleppo. More than 500,000 people have died since war began in 2011. Imagine it. Years of cluster bombs, rockets and toxic gas raining down, funeral after funeral, death after death, while the world looks on and watches your country and your people being destroyed.

Only our leaders can stop the slaughter of civilians in Aleppo and across Syria

Every time we thought it couldn’t get worse, it did. And now, again, the horror is plumbing new depths. Families are running out of food, of water, of medicine. There are no more operational hospitals and not a single ambulance to rescue the ever-growing number of wounded and sick. In this city of death, even the clowns are dying. The man who tried to block out the horror by entertaining the children, distracting them for a moment, was killed. Was this the death of hope?

I hope not, as I have been there, staring death in the face, desperately alone. On a night in mid-July 1995, Serbian soldiers took us to the field where we were to be executed. They stripped us and tied our hands behind our backs. Lined up, five by five, rows and rows of dead bodies before my eyes, already shot, lives snuffed out in a single bloody instant. I was shot in my stomach, right arm and left foot and felt an incredible pain, as the gasping last breaths of men filled my ears. When the butchers left, I realised I was not dead and managed to escape with another man. For days we kept running, hiding in the woods and sleeping in graveyards until we eventually reached the safety of Bosnian-controlled territory. I wondered then how the world could let this happen.

In 2005, the European parliament released a statement, condemning the Srebrenica genocide and promising it would “never happen again”. It gave me hope, that what we went through had not been in vain and we had learned from the horrors of the past. In future, I thought, the international community would protect civilians in times of conflict. That seems a long time ago now.

I have faith in humanity. In people all over the world. I know most of them would help the people of Aleppo if they could. But they can’t do it alone. Only our leaders can stop the slaughter of civilians in Aleppo and across Syria. At the very least, they must ensure aid gets to those who need it, including by airdrops to besieged areas if necessary. Their failure to do so is a betrayal not only of the people of Aleppo and Syria, but of the survivors and victims of all the genocides we said we’d learned from.

Instead, the worst of humanity has become the new normal. When we look the other way, we set the most dangerous of precedents, one that makes my experience more likely to be repeated. I’ve looked down the barrel of the gun, and know that humanity can’t afford this. Take it from a genocide survivor – more than Aleppo is at stake.