The North Atlantic Treaty was signed in 1949. However, Article 5 was neither triggered in 1961 when the Portuguese Colonial War started in Angola, nor after the Gulf of Tonkin Incident (1964) that started the (overt) U.S. involvement in the Vietnam War.
Portugal, as a founding member nation of NATO, could have activated Article 5 for U.S. and U.K. engagement in the war, and perhaps they could have kept their African colonies till today. Instead, the long lasting war triggered the April 1974 coup d'état in mainland Portugal, led to the collapse of the Estado Novo (the 2nd Republic) and its African colonial empire.
The U.S. was attacked by North Vietnam, and Portugal suffered from Cuba's intervention in Angola. Both NATO members were attacked by sovereign nations. Why, in these circumstances, was not only Article 5 not triggered, but also NATO as a military alliance did nothing to help?