The 20th anniversary of the worst terrorist attacks in the US comes when the withdrawal of US troops from Afghanistan is fresh in the mind and its consequences are still incalculable. Trends Wide diplomatic editor Nic Robertson was in Kabul when al Qaeda struck the United States on September 11, 2001. Today, Robertson looks back at the past and the effects of two decades of a war that toppled the Taliban, and sought to prevent attacks. like those that happened 20 years ago. Now, America’s longest war is coming to an end and, with the Taliban once again in command of Afghanistan, the threat against the US is on the surface.