On 12 September, NATO decided that, if it is determined that the attack against the United States was directed from abroad, it shall be regarded as an action covered by Article 5 of the Washington Treaty.
This is the first time in the Alliance's history that Article 5 has been invoked.
NATO lost its way when the Balkan operation began. I still don't know which NATO nation was attacked by Serbia. Anyone?
Nevertheless even in Afghanistan, when clearly a NATO country was attacked, the response by most signatories was tepid. Traditional US allies went in heavy, the rest, not so much.