This document discusses NATO's response to the September 11th terrorist attacks. It covers NATO's history as a political and military alliance formed after World War II. On September 11th, 2001 the United States invoked NATO's Article 5 for the first and only time, treating the terrorist attacks as an action against all NATO members. NATO supported the US-led war on terrorism and expanded its mission beyond territorial defense of member states. The document examines the key decision makers, outcomes, and implications of NATO's response to 9/11 both in the short and long term.