Kassem Eid will never forget the events of 21 August 2013. He awoke unable to breathe. His eyes were burning and there was a drumming pain in his head. As he stumbled outside, he saw a small boy lying in the street, his “innocent face stained with grotesque shades of red, yellow and blue”. There was white vomit around his mouth and he was struggling to breathe. Eid shouted for help, “begged Allah for mercy”, but there was nothing anyone could do.

This was “chemical day”, when President Assad’s troops attacked rebel-held suburbs of Damascus with sarin gas, killing as many as 1,500 people. It happened while UN chemical weapons inspectors were in the Syrian capital.

Eid was living in Moadamiya, one of the areas besieged by Assad’s forces. That day “changed me forever”, he writes in this powerful memoir. It was the day that he took up arms against Assad.

As a Palestinian-Syrian and Sunni Muslim, Eid faced constant discrimination. In 2011 he took part in demonstrations against the regime and experienced the brutality of Syria’s feared secret police, the Shabiha. An English speaker, he spread the news of Assad’s savagery to the outside world using the pseudonym Qusai Zakarya, a role that eventually took him to Lebanon and then the US.

But, disillusioned by politicians’ refusal to help his country and tormented by guilt for having escaped the continuing conflict, he tried to return to Syria. Now a refugee in Germany, he is understandably angry: “I can’t believe that humanity has become so deaf and blind to the thousands of Syrians with stories like mine.”

This is a humbling and important first-hand account of a brutal civil war in which as many as 500,000 people have died.