On this day,15 years ago, I joined about one million others in central London to march against the impending Iraq war. We were not just part of the biggest political demonstration in British history, but the largest simultaneous protest event in the world, with millions marching in every continent. We gathered in Hyde Park and heard many famous voices speaking out against the war, from Labour politician Tony Benn to playwright Harold Pinter.

When Jeremy Corbyn took the stage, he too was a familiar face to all peace march veterans and Islington residents like myself, but less so to other members of the crowd. But his words stirred the souls of everyone present. He asked why we could afford to spend billions on a war nobody wanted while children around the world were dying of poverty and starvation. And he warned of the risks in invading Iraq: “It will set off a spiral of conflict, of hate, of misery and of desperation that will fuel the wars, the conflict, the terrorism, the depression and misery of future generations.”

Ask the British government what it is doing to help stop this awful slaughter and you just get a shrug

How terrifyingly right he was. And again when he made the same arguments in respect of Libya and Syria. In the 2011 debate on Libya, as a backbencher under Ed Miliband, Corbyn warned: “We have not thought through the implications of what we are doing.” I for one should have listened, rather than obeying the Labour three-line whip.

And as party leader, his speech in the 2015 Syria debate – pilloried by the Tory government and Labour critics at the time – now reads like the cries of Cassandra. Corbyn warned that with no credible Syrian opposition forces, no long-term strategy and no plan for a political settlement, the refugee crisis and civilian casualties would only grow, and mission creep was inevitable. “Is it right,” he asked, “for us here in Westminster to see a problem, pass a motion and drop bombs, pretending we are doing something to solve it? That is what we did in Afghanistan, Iraq and Libya. To oppose another war and intervention is not pacifism; it is hard-headed common sense.” Instead, he said, all Britain’s efforts should be directed to producing a peaceful and just political settlement to the war.

More than two years on, that goal is further away than ever, even though the justification for the US-led coalition’s intervention in Syria – denying Islamic State a haven – was achieved months ago. Just in the past four weeks, we have seen unprecedented levels of escalation, aggression and land-grabbing by the competing foreign powers enmeshed in this terrible war.

The US announces plans for an “open-ended” occupation of northern Syria; Turkey launches a brutal assault on Kurdish-held areas; Russian and coalition bombers trade devastating blows against each other’s proxy armies; Israel launches its biggest air strikes inside Syria for 36 years in response to Iran sending a drone across its borders; and the Gulf states continue to feed money and weapons to the dwindling opposition and jihadist militias.