The ghostly silence and calm that settled over Washington, DC is what many in the city remember most about the aftermath of the 9/11 plane hijackings and ensuing destruction of the Twin Towers and a portion of the Pentagon.

City streets were emptied of traffic. The airports closed and most public transit came to a halt. Not since British troops burned the Capitol and the White House during the War of 1812 had there been an attack of that magnitude on mainland America.

The threat has not gone away. It has since morphed into smaller-scale events carried out by lone actors

Virtually all of the nation’s elected leaders and much of its bureaucratic workforce were evacuated. It was assumed there would be further violence, and for months afterward the nation’s policy and political establishment focused on ways to defend against more devastating assaults: a chemical weapons attack, a suitcase nuclear bomb, biological weapons, vulnerable city reservoirs that might be poisoned. Nightmare scenarios multiplied.

It was 25 years ago to the day that the towers fell, causing immeasurable changes to life in the US and in Europe. Two wars ensued, three if you count the “war on terror,” as well as the wholesale restructuring of airport security, intelligence gathering, law enforcement tactics, foreign policy and immigration law.