For those who would rather not wear Tin Foil hats, here is the text from a Brian Dunning podcast (of Skeptoid fame) from 2009:
Today we're going to turn the pages back to an American UFO story dating from World War II, the Battle of Los Angeles, when (according to modern lore) the United States Army and Navy battled a giant UFO hovering above the city of Los Angeles.
It was late February, 1942, less than three months after the Japanese attacked Pearl Harbor. Residents on the Western coast of the United States expected they were next, and so stood ready with hasty fortifications and kept their eyes on the sky.The crews manning the antiaircraft artillery batteries in Los Angeles had been trained, but lacked experience in actual combat. Only one day before, the Japanese submarine I-17 had surfaced off of Santa Barbara and fired 25 shells at some aviation fuel storage tanks, so the alert level was the highest it had ever been. An attack on Los Angeles was imminent.
Just after 2:00am on the morning of February 25, radar picked up a target off the coast. The antiaircraft batteries in Los Angeles were put on Green Alert, ready to fire. By 2:21am the radar target had approached closer, and a blackout was ordered. The radar lost contact with its target, and searchlight beams swept the sky for nearly half an hour. Then, reports of aircraft came in. Over Santa Monica, a balloon carrying a red flare was spotted, and the batteries opened fire at 3:06am. The Battle of Los Angeles was on.
For nearly an hour, batteries fired 1,430 rounds of antiaircraft artillery, raining eight and a half tons of shrapnel back down onto Los Angeles. But what did they see? What were they shooting at? Therein lies the rub. Many saw nothing. Some reported balloons. A few reported airplanes. CBS Radio called it a blimp. The moon had set at 2:30am, and sunrise was not until 6:30am; combined with the blackout, it was about as dark as dark can be. The only thing anyone could see was whatever the searchlights struck, which was smoke from the AAA bursts. The Office of Air Force History described the field reports as "hopelessly at variance". The most famous photograph, from the Los Angeles Times, shows a convergence of searchlights onto a single large cloud of smoke. Property damage from the shrapnel was widespread, and since no bombs were dropped and no evidence of enemy aircraft was ever discovered, demands for explanations and investigations followed: Both in a scathing editorial in the Los Angeles Times the following day, and from the White House.
Secretary of the Navy, Frank Knox, held a press conference that same day to state that it was a false alarm, that no aircraft had been involved, and that the entire incident had been an expensive case of jittery nerves. Chief of Staff George Marshall wrote a memo to President Roosevelt, stating the current understanding that airplanes may or may not have been involved, possibly as many as fifteen, possibly commercial aircraft, at various slow speeds. Given the lack of confirmation that any aircraft were present at all, Roosevelt's response was to ask the Secretary of War to clarify exactly who is authorized to order an air alarm.
And that's where the story was left for decades: a false alarm from the opening days of World War II: No mysteries, no strangeness, no aliens, no supernatural element. But of course, as you can guess, it did all eventually appear. It took more than 40 years, but UFO enthusiasts finally decorated the Battle of Los Angeles with some imaginative additions.
To understand how it happened, you first have to understand the Majestic 12 papers. In 1987, a group of UFOlogists, William L. Moore, Stanton Friedman, and Jaime Shandera, announced the existence of several government documents, classified as top secret, that purported to contain a 1947 order from President Harry Truman establishing a group called Majestic 12, an assortment of the usual Illuminati from government, business, and the military. Majestic 12 was charged with handling everything to do with extraterrestrial aliens.
Later, another UFOlogist, Tim Cooper, announced his own batch of secret Majestic 12 documents. Rival UFOlogists work together in the same way that rival Bigfoot hunters do: Not very nicely. Moore and his proponents launched into Cooper's documents, pointing out clues that prove them counterfeit; and Cooper and his proponents did the same to Moore's documents, revealing the flaws that disproved their authenticity. When infighting among adversarial bamboozlers does all the work revealing each others' hoaxes, it makes the legitimate investigator's job so much easier.
Among this tangled mess of hoax documents is a letter called the Marshall/Roosevelt Memo from March 5, 1942, stating that two unidentified aircraft were in fact recovered after the Battle of Los Angeles: One at sea, and one in the San Bernardino Mountains east of Los Angeles. It says in part:
This Headquarters has come to a determination that the mystery airplanes are in fact not earthly and according to secret intelligence sources they are in all probability of interplanetary origin.
The letter is, of course, properly scuffed up and smudged in the most realistic and dramatic fashion. A PDF of it is available for download from MajesticDocuments.com. Hilariously, page 2 of the PDF is an order form to purchase a wide range of UFO related documents, CDs, and books. Obviously, it's not legal to distribute actual top secret documents, and the fact that the FBI permits the availability of this (and the many others on MajesticDocuments.com) is a pretty good tipoff to the FBI's assessment of their authenticity. Skeptical investigator Philip Klass brought the documents' publication to the FBI's attention in 1988, and the FBI quickly concluded that all the documents were fake. So download freely, and send in those order forms.
As far as I could determine, this letter's late-1980's appearance was the earliest reference to anything UFO related happening at the Battle of Los Angeles. Since then, of course, innumerable references have appeared on the web. Most UFO websites discuss the battle and show the picture from the LA Times, describing the cloud of AAA smoke in the searchlights as a "large craft". But this was not the contemporary identification. For more than 40 years, not a single person associated with the Battle of Los Angeles entertained any thoughts about extraterrestrial spacecraft or aliens, according to all available evidence (at least when you discard the hoaxed evidence). The alien spacecraft angle is purely a post-hoc invention by modern promoters of UFO mythology.
Modern UFOlogists seem to have forgotten what the "U" in UFO stands for: Unidentified. They tend to identify such objects as extraterrestrial spacecraft, for reasons known only to themselves; so they should really pick a new term. The Battle of Los Angeles was triggered by true UFO's: Something spotted in the sky that nobody was able to definitively identify. Most gunners reported never seeing anything at all, and simply fired at wherever they saw other air bursts. For this, the gun crews were officially reprimanded. The Office of Air Force History says in its 1983 report entitled The Army Air Forces in World War II:
A careful study of the evidence suggests that meteorological balloons — known to have been released over Los Angeles — may well have caused the initial alarm. This theory is supported by the fact that anti-aircraft artillery units were officially criticized for having wasted ammunition on targets which moved too slowly to have been airplanes. After the firing started, careful observation was difficult because of drifting smoke from shell bursts. The acting commander of the anti-aircraft artillery brigade in the area testified that he had first been convinced that he had seen fifteen planes in the air, but had quickly decided that he was seeing smoke. Competent correspondents like Ernie Pyle and Bill Henry witnessed the shooting and wrote that they were never able to make out an airplane.
But of course, to the conspiracy theorists and UFO believers, any report put forth by the Air Force is simply part of the conspiracy and not to be trusted. So let's play the devil's advocate and assume that interplanetary spacecraft were, in fact, shot down during the battle and recovered, and the government has full knowledge of it, as the UFOlogists expect us to believe. Then it becomes a question of how they were able to keep this a secret for more than 40 years: Retroactively change the newspaper accounts, change the radio reports, pay off or kill everyone who participated, pay off or kill everyone in Los Angeles who witnessed it, yet continue to allow the "top secret" confessions to be downloadable from the Internet; the proposition quickly becomes ludicrous.
An alternate explanation, supported by evidence, requires us to make no such absurd leaps of logic or pseudoscientific assumptions: That the Battle of Los Angeles was simply a case of jittery nerves, at a time when every single person in Los Angeles was living in daily fear for their lives from imminent Japanese attack. There is simply no need for the introduction of a paranormal element to explain it. Whenever you hear a tale from history that involves alien spacecraft or any other paranormal element, you should always be skeptical.