September 11th attacks: 25 years on

Twenty five years ago, America was under attack.

Within days, NATO triggered Article 5 of the North Atlantic Treaty, without a specific request from the US.

Article 5 states that an attack on one member of NATO is an attack on all. It allows each member state to help by taking any necessary measures, including armed force, to restore and maintain the security of the North Atlantic area.

This is the only time since NATO was established that Article 5 has been invoked.