NATO after 11 September

On the evening of 12 September 2001, the members of the North Atlantic Treaty Organisation invoked that treaty's mutual defence guarantee for the first time in the alliance's 52 years. When that treaty's Article 5 was drafted - pledging that an attack on one ally would be treated as an attack on all - not a single signatory could have imagined that its first invocation would involve Europeans coming to the aid of the United States rather than the other way around. Yet that is precisely what happened, and NATO will never be the same again. The notion that mutual defence could be a two-way street, and that NATO might use its military power to deal with international terrorism - in Central Asia no less - are just some of the ways that the attacks have begun to transform the world's largest and longest-standing defence alliance.