According to Wikipedia, the Navy let loose a barrage at what they thought was another Japanese attack force but the object(s) they fired upon were never determined. The assault created a panic in the city as the media tried to capture another potential attack on U.S. soil. Later the Secretary of the Navy, Frank Knox declared it a false alarm and bad case of "war nerves." After all, this was a mere two months after the bombing of Pearl Harbor. After further investigation from the Air Force, they declared it was probably an errant weather balloon.
This was the 1940's and science fiction had taken a hold on the culture. Orson Wells created pandamonium throughout the country with his now infamous War Of The Worlds radio drama only four years prior, and apparently rouge military weather balloons were en vogue. It didn't take long for true believers to change the focus of the unknown threat chronicled that day from the Japanese to a more mysterious, extra terrestrial nature. This was five years before the Roswell incident, so there was no reported or documented evidence of the existence of UFOs or aliens. Bill Birnes, co-author of the book The Day After Roswell" and Dr. Bruce Maccabee, a leading Ufologist recall the events of this day in L.A. and make the argument the item being fired upon that day was probably of alien origin in a featurette titled,Battle: Los Angeles News Wrap on the upcoming movie's official website.
Battle: Los Angeles opens in theaters on Friday, March 11.