I don't believe in UFOs as valid weapons or spy tools of the US or any other government, worldly or not. Very few of the UFO accounts described obey the laws of basic physics, witnesses project their own personal prejudices and localisms onto their accounts, and a quick brush-up with any physical sciences resource should show that the UFO is little more than a final holdout from the 1950s or a favorite hobgoblin of the conspiracy theorist.
The weapons the world has, from nukes to EMP weapons to flying drones, make such things obsolete.
As for the other aspect of UFOs, the alien factor, I think that alien creatures possessing similar levels of technology are so far away that any spaceships attempting to reach us have outlasted their creators' civilizations... and run out of power or been destroyed, as well.
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On 16 July 1969, a United States law was passed called the "Extra-Terrestrial Exposure Law" that made it illegal for the public to come in contact with extra-terrestrials or their vehicles. (Title 14, Section 1211 of the Code of Federal Regulations). Anyone found guilty of such contact could face up to one year imprisonment as well as a fine of $5000. Also,any individual who had been "exposed" could be quarantined under armed guard by the NASA administrator without a hearing.The law was passed originally to protect Earth from possible biological contamination resulting from the US Apollo Space Program and other space exploration programs. It has been suggested by researchers and scientists that the U.S.government was very concerned that contact with extra-terrestrial bacteria could result in a worldwide plague. The immune system of human beings wouldn't be able to fight off extra-terrestrial bacteria, so therefore any kind of "extra-terrestrial exposure" was taken very seriously. People in the UFO community especially were alarmed with the law because its broad definitions could allow the US government to prosecute people in NASA as well as individuals of the general public who come into contact with extra-terrestrials and their vehicles.
UFO and ET believers found it interesting that in one hand the
government was denying they had any interest in extra-terrestrials, yet they had a law which forbid contact with ETs and their craft.
On 7th August 1996,NASA (1) scientists made an announcement that made
front page headlines throughout the world.Within a Martian
meteorite,they had found evidence of a microscopic lifeform that may
have existed on Mars more than three billion years ago.