Gearing up for the release of next year's Louisiana-shot alien movie, Battle: Los Angeles 2011, Sony launched the flick's website, complete with authentic stories on the real life "Battle of Los Angeles." As reported, the American military went into hyper-drive during the early hours of February 25, 1942 when unidentified objects were spotted in the night sky; sirens went off, the city went dark, and the army blasted 1,500 rounds of anti-aircraft artillery into the air: three people died from falling shells, three others from stress-induced heart attacks due to the hour-long bombardment. Sony's movie goes with the premise that it was aliens, as opposed to Japanese aircraft, biplanes, or weather balloons, that triggered the army.
The Battle website has three articles concerning the events of February 1942. The first story is a 1945 Los Angeles Times piece that states the Army still isn't clear what was spotted in the sky before the air-raid sirens went off. The piece includes a rumor that Japanese planes may have taken off from secret Mexican bases. Also of note was that a Japanese submarine fired on oil installations near Santa Barbara just a few days before the Battle of Los Angeles, which may have been why officials were a bit jumpy on February 25th.
An Associated Press article from 1945 (reprinted twice on the website) indicates that one to five unidentified planes flew above LA that night; a lieutenant general said "three planes appeared over Beverly Hills." That same official stated that he believed those planes were launched from enemy submarines (what? scary!).
The last article is from the LA Times again, and looks back at the 25th anniversary of the events. Here's their description of events: "The night of Feb. 24-25, at 2:23 a.m., the air raid sirens began to shriek. Madness was loosed. The city was blacked out, searchlights stroked the sky. Antiaircraft guns opened up. The night was laced with trackers and explosions. In the streets citizens drove automobiles into one another."