It had taken just a few weeks to mobilise the huge international operation but it wasn't fast enough. One exit route had already become too dangerous - in the previous days it had fallen under the control of Islamic State - and half of those they hoped to rescue didn't make it out. It is not clear what happened to these 400 people, but they were advised to burn their uniforms and hide. Senior members of the White Helmets told me they believe many of those left behind were captured and tortured or killed.