In the aftermath of the disastrous war in Iraq, the lesson seemed clear: the West should never intervene in foreign conflicts. But then came the Syrian civil war, and the invasion of Ukraine, and the withdrawal from Afghanistan. So 20 years on, Caroline Wyatt – who has reported from Iraq, Afghanistan and Russia – takes us back to the choice of horrors the West faced in 2003, and examines how the legacy of that fateful decision shapes foreign policy today, for good or ill.