Another possibility is that what we see as UFOs are actually the
result of advanced atomic powered aerospacecraft developed by the
USA.  The UFO legends are then used to hide the reality of these
aerospacecraft in plain sight.

http://en.wikipedia.org/wiki/Nuclear_aircraft
http://en.wikipedia.org/wiki/Project_Pluto

Also, what we see as aliens are men who have been re-engineered into
cybernetic organisms to operate in space indefinitely without
conventional life support or other concerns a non-engineered human
would incur - all by the USA.  Again, UFO legends are used to hide the
reality of these cybernetic organisms.

http://en.wikipedia.org/wiki/Cyborg

The use of this set of assets by the USA include;

 (1) the ability to discredit someone at will by creating a UFO
experience (this was one of the goals of MKULTRA program)

See goal 1
http://en.wikipedia.org/wiki/Project_MKULTRA#Goals

 (2) the ability to invoke shock and awe in a post national situation
and maintain the ability to influence on global affairs by pretending
to be 'superior' aliens as a means to implement COG (continuity of
government) following a national or international catastrophe;

http://en.wikipedia.org/wiki/Continuation_of_government