Not only those WWII contacts corrupted hitherto "sheltered" cultures: one only needs to look back to what the Spanish did to South American cultures, or even farther back to Roman conquests, to see how unbalanced such contacts are.
However, we 're talking about a non-human entity, potentially, whose main attributes we share in common might be their curiosity and their intelligence. Other than that curiosity, maybe they wouldn't have human-like emotions, neccesarily, and thus would not be shackled by such flawed chracteristics as those of our greed, loathing and hatred.
Perhaps, if First Contact has taken place already, Govts and the military are, obviously, already aware of such fundamental differences between "Us and Them"; but their fear of Humanity's likely general initial reaction far outweighs, notwithstanding, genuine inside awareness of the alien entities' evident impartiality. For, make no mistake, if there has already been such Contact, any such entities would already have been able to dominate and control us long ago. But they haven't.
So, Govts' next task must be to educate any potential panic reactions out of mainstream public thinking and groom the group psyche for acceptance of this news, when it breaks. Perhaps it's started already, with a "nod and a wink" from Govts and the resultant steady drip-feed of Sci Fi movies and TV series like Star Trek and The X-Files.
But maybe Govts have lost their nerve, again - if they ever really hadn't (!) . For how long, and what, will it take before the Announcement, which many are predicting, finally comes ?