Proof of the UFO/ET coverup

Evidence exists that proves the U.S. government and other world governments have been keeping secret the reality that there is an extraterrestrial presence on Earth. A 1950 document written by a Canadian official with regard to what the U.S. knew at that time is just one piece of the evidence, but it speaks volumes. Also, if the government was to disclose the truth finally, how would it achieve that purpose?