On the night of February 24, 1942, an incident that has come to be called "The Battle of Los Angeles" erupted in the California sky. It happened less than three months after the U.S. had entered WW2.
An unidentified flying object was spotted, air raid sirens sounded, and a blackout was ordered. By the end of the "battle," over 1400 anti-aircraft shells (12.8 pounds each) had been fired at the object but the 37th Coast Artillery Brigade could not bring it down.
Several buildings were damaged by friendly fire, three civilians were killed by the anti-aircraft fire, and another three died of heart attacks attributed to the stress of the hour-long bombardment.
(I adapted this information from Wikipedia.)
Below are three videos about this incident. Please note that there are many more videos about The Battle of Los Angeles on YouTube if you care to search.
Following is an excerpt from a very informative, highly detailed web page containing multiple historical source reports about this incident. Follow the link at the bottom to read the rest.
THE TRUTH OF WHAT HAPPENED on that clear, black night nearly seven decades ago will likely never be known. It now exists only in faded memory and vivid imagination, and as a historical curiosity -- with ample room to support a variety of conclusions and beliefs.
In the end, the only thing that can be safely stated is that if there was something up there then what was then called in war parlance an 'unidentified aerial target' was the first modern, well-reported incident of what is now known as an 'unidentified flying object', or 'UFO'.
And that it (or 'them' if the reports of planes are to be believed) showed remarkable resilience in the face of an onslaught of more than 1400 anti-aircraft shells lobbed with deadly intent into the sky.
Source: Saturday Night Uforia