This is in remembrance of the terrorist attacks of September 11th 2001 on the United States which sent shockwaves throughout the world and NATO was no exception.
Sixteen years after that historic day, NATO has move to its new headquarters and in the new building, this key date is commemorated by displaying a piece of the Twin Towers.
The day after the 9/11 attacks, NATO invoked Article 5 for the first time in its history.
It was the first time that the Alliance invoked one of its founding principles, that an attack against one Ally is considered as an attack against all Allies.
This decision marked a turning point in the Alliance, which found itself facing multiple new threats.