When we step back for a moment - consider this: The US Military doesn't make diddly siht. Centuries ago, the US had a couple of armories - but that's it - and even those were abandoned.
When a new weapon system is developed - a rifle - a pistol - a helicopter - a grenade launcher - a jet or cargo plane - warship - it's always developed and built by civilian companies. They may work with the military to determine needs, specifications, capabilities, and even funding - but still made by non-government companies.
The Federal Government is not in business. They buy things they need - they don't produce them.
So, I'm in the military and found a flying saucer - complete with little guys that got squashed and busted up in the crash.
What to do with it? It obviously has technology we don't have - so who do we get to disassemble, evaluate, and determine what these systems are capable of and can we duplicate them?
Let's say it runs off a power system very powerful and the fuel is a small amount of Unobtainian - and and that creates a powerful spinning outer ring of pure mercury that is capable of blocking and attracting gravitational effects.
Not only that, but the outside of the craft - via ultra-high voltage ionization enables everyone inside - to disregard classical physics.
You want to keep this power supply/source to yourself. If suddenly there's pretty much free, unlimited energy - then suddenly there are untold trillions of dollars of dollars in energy-specific assets and systems world-wide - and they are suddenly obsolete. Literally millions of jobs - gone. The crash would be catastrophic. All around the world.
No more mass trucking required - nor all those support personnel - millions and millions of folks. Air transportation - obsolete. Auto manufacturing - crippled. Like Route 66 - many roads bypassed. Shipping - no longer needed. More millions of jobs - gone.
The little people inside - recall the Vatican sending a rep to Galileo - to tell him to knock it off. Galileo says there are craters on the moon - and the Vatican rep refutes him. Galileo says, here's my telescope - look for yourself!
His reply? "I dare not look lest I become like you."
The affect on world religions would be very upsetting. We humans would go from a belief that all the heavens rotates around the Earth - Earth being the center of the Universe - to Heliocentric realization within our Solar System, and we're shocked to find that we're just another planet - and now - we're slapped in the face by discovering we're just another species in the Creation.
Oh, doo-doo.
That's the other side of the argument. Both have some merit. But we seem to adapt to new discoveries fairly well, so let's open it up! We can go to Mars - not in three months - but in three hours. Everyone has energy wherever they live.
Could end up being bad, but we have to adapt eventually - so why not now?