Also known as The Great Los Angeles Air Raid, this might be one of the most widely-reported and still unsolved mysteries in modern history. The facts, such as they are, are these: from late 24 February to early 25 February 1942, the US military responded to an unprovoked, unidentified attack by enemy forces above the skies of California.
The incident came a scant few months after the Pearl Harbour attacks, meaning America had just entered the war and were a little jumpy. Still, they wouldn't unleash a barrage of anti-artillery fire on absolutely nothing. At first it was assumed to be a second attack by Japanese forces, but that was swiftly debunked.
Secretary of the Navy Frank Knox waved it away as a false alarm, but didn't explain the bright lights in the sky which caused the army to fire in the first place. UFOlogists assert that this was one of the most significant paranormal events in history: first contact. Or rather, first contact, and the aliens were absolutely not friendly.
Even if the Battle Of Los Angeles wasn't the first time humans and extraterrestrials met, the event at least had another tangible effect on history: since then, reports of UFO sightings, and the military's interactions with which, have increased exponentially...