When told: "aliens are a national security threat", this is a cover-up for the fact that there are illegal operations taking down actual UFOs and reverse-engineering them since before the 1950s, by major contractors but outside of any government control. These people are trying to influence government that aliens are somehow a threat, to justify their actions. The reality is that highly advanced civilizations that make it here would never be interested in violence – and that is our next step as a civilization as well. To raise our consciousness and become a non-violent species.
Check out Dr. Steven Greer's work on this. He has written great books and made great documentaries, alongside briefing every president on the subject since Clinton.