In the frantic weeks that followed the Pearl Harbor attack, many Americans believed that enemy raids on the continental United States were imminent. On December 9, 1941, unsubstantiated reports of approaching aircraft had caused a minor invasion panic in New York City and sent stock prices tumbling.

On the West Coast, inexperienced pilots and radar men had mistaken fishing boats, logs and even whales for Japanese warships and submarines. Tensions were high, and they only grew after U.S. Secretary of War Henry Stimson warned that American cities should be prepared to accept “occasional blows” from enemy forces.

Just a few days later on February 23, 1942, a Japanese submarine surfaced off the coast of Santa Barbara, California, and hurled over a dozen artillery shells at an oil field and refinery. While the attack inflicted no casualties and caused only minor damage, it marked the first time that the mainland United States had been bombed during World War II.

The day after the oil field raid, paranoia and itchy trigger fingers combined to produce one of the most unusual home front incidents of the war. It began on the evening of February 24, 1942, when naval intelligence instructed units on the California coast to steel themselves for a potential Japanese attack.

All remained calm for the next few hours, but shortly after 2 a.m. on February 25, military radar picked up what appeared to be an enemy contact some 120 miles west of Los Angeles. Air raid sirens sounded and a citywide blackout was put into effect. Within minutes, troops had manned anti-aircraft guns and begun sweeping the skies with searchlights.

It was just after 3 a.m. when the shooting started. Following reports of an unidentified object in the skies, troops in Santa Monica unleashed a barrage of anti-aircraft and .50 caliber machine gun fire. Before long, many of the city’s other coastal defense weapons had joined in.

“Powerful searchlights from countless stations stabbed the sky with brilliant probing fingers,” the Los Angeles Times wrote, “while anti-aircraft batteries dotted the heavens with beautiful, if sinister, orange bursts of shrapnel.”

Chaos reigned over the next several minutes. It appeared that Los Angeles was under attack, yet many of those who looked skyward saw nothing but smoke and the glare of ack-ack fire.

“Imagination could have easily disclosed many shapes in the sky in the midst of that weird symphony of noise and color,” Coastal Artillery Corps Colonel John G. Murphy later wrote. “But cold detachment disclosed no planes of any type in the sky—friendly or enemy.”

For others, however, the threat appeared to be very real. Reports poured in from across the city describing Japanese aircraft flying in formation, bombs falling and enemy

paratroopers. There was even a claim of a Japanese plane crash landing in the streets of Hollywood. “I could barely see the planes, but they were up there all right,” a coastal artilleryman named Charles Patrick later wrote in a letter. “I could see six planes, and shells were bursting all around them. Naturally, all of us fellows were anxious to get our two-cents’ worth in and, when the command came, everybody cheered like a son of a gun.”

The barrage eventually continued for over an hour. By the time a final “all-clear” order was given later that morning, Los Angeles’ artillery batteries had pumped over 1,400 rounds of anti-aircraft ammunition into the sky.

It was only in the light of day that the American military units made a puzzling discovery: there appeared to have been no enemy attack. “Although reports were conflicting and every effort is being made to ascertain the facts, it is clear that no bombs were dropped and no planes were shot down,” read a statement from the Army’s Western Defense Command.

Ironically, the only damage during the “battle” had come from friendly fire. Anti-aircraft shrapnel rained down across the city, shattering windows and ripping through buildings. One dud careened into a Long Beach golf course, and several residents had their homes partially destroyed by 3-inch artillery shells.

While there were no serious injuries from the shootout, it was reported that at least five people had died as a result of heart attacks and car accidents that occurred during the extended blackout. In a preview of the hysteria that would soon accompany the Japanese internment, authorities also arrested some 20 Japanese-Americans for allegedly trying to signal the nonexistent aircraft.

Over the next few days, government and media outlets issued contradictory reports on what later became known as the “Battle of Los Angeles.” Secretary of the Navy Frank Knox dismissed the firefight as a false alarm brought on by “jittery nerves,” but Secretary of War Henry Stimson echoed Army brass in saying that at least 15 planes had buzzed the city. He even advanced the provocative theory that the phantom fighters might have been commercial aircraft “operated by enemy agents” hoping to strike fear into the public.

Stimson later backpedaled his claims, but there was still the matter of the thousands of military personnel and civilians who claimed to have seen aircraft in the skies over L.A. According to an editorial in the New York Times, some eyewitnesses had spied “a big floating object resembling a balloon,” while others had spotted anywhere from one plane to several dozen. “The more the whole incident of the early morning of Feb. 25 in the Los Angeles district is examined,” the article read, “the more incredible it becomes.”

What caused the shootout over Los Angeles? The Japanese military later claimed it had never flown aircraft over the city during World War II, providing fuel for a host of bizarre theories involving government conspiracies and visits by flying saucers and extraterrestrials.

Still, the most logical explanation for the firefight is that trigger-happy servicemen and rudimentary radar systems combined to produce a false alarm. In 1983, the Office of Air Force History outlined the events of the L.A. air raid and noted that meteorological balloons had been released prior to the barrage to help determine wind conditions. Their lights and silver color could have been what first triggered the alerts. Once the shooting began, the disorienting combination of searchlights, smoke and anti-aircraft flak might have led gunners to believe they were firing on enemy planes even though none were actually present.

While it’s likely that the Battle of Los Angeles was only a mirage, it was still a chilling reminder of the vulnerability that many Americans felt at the beginning of World War II. The Japanese would later hatch several schemes to attack the American mainland—including launching over 9,000 explosives-laden “fire balloons”—yet none of them ever produced the level of mass hysteria that accompanied the phantom shootout over Los Angeles.

Even at the time, many journalists noted that it was fitting that the incident had taken place in the home of the film industry. In an article from March 1942, the New York Times wrote that as the “world’s preeminent fabricator of make-believe,” Hollywood appeared to have played host to a battle that was “just another illusion.”