Final answer:
Article 5 of the North Atlantic Treaty was invoked for the first time in response to the September 11, 2001, terrorist attacks on the United States.
Step-by-step explanation:
The first time Article 5 of the North Atlantic Treaty had to be invoked was in response to the September 11, 2001, terrorist attacks on the United States. This collective defense clause assured NATO members that an attack on one would be viewed as an attack on all. Following 9/11, NATO members supported the United States' actions in Afghanistan, where they attacked the
Taliban government and aided in stabilizing the country under a United Nations-authorized, NATO-led peace-building force.