Why should the USA send troops to Europe? I'll remind you that article 5 of NATO was only invoked once: when you called for help to invade one of the poorest nations on earth: Afghanistan. Could the wealthy American democracy not defend itself?
On the evening of 12 September 2001, less than 24 hours after the attacks, the Allies invoked the principle of Article 5. Then NATO Secretary General Lord Robertson subsequently informed the Secretary-General of the United Nations of the Alliance's decision.