Has anybody taken into concideration that the alarm was actually real? That there was really a missile launched towards Hawaii (or the US coast), but that it was intercepted.
In such a case there are 2 options: to tell the public it was a false alarm and pretend nothing happened, or to declare war on the country that launched the missile.
Lets say this country was China, or rogue elements in the chinese army/navy. That would be a good enough reason to keep things low profile.