One of the first notable examples of Hollywood’s depiction of the UFO phenomenon is The Day the Earth Stood Still (1951), adapted from Harry Bates’ 1940 short story “Farewell to the Master.” In the film, a flying saucer causes utter chaos when it appears in the skies over Washington, DC. Touching down outside the White House, a British-accented alien named Klaatu emerges and asserts that he means only goodwill towards humankind; he wants to gather the world’s leaders together to deliver an important message.

Rebuffed by suspicious U.S. authorities, Klaatu befriends Helen and her young son, who introduce him to a prominent scientist, Professor Barnhardt. When Klaatu is shot and killed by the military, only Helen is able to give a key order to Klaatu’s faithful robot servant, Gort, in order to resurrect his master. Alive again, Klaatu is finally able to deliver his message to mankind: The development of atomic weapons on Earth has been noted by the Galactic Federation, which will not stand for their misuse. The mighty Gort will serve as a planetary policeman, with the authority to destroy the world if things get out of hand.

The Day the Earth Stood Still and its rather pessimistic ending–according to Klaatu, the Earth has only two choices: live in peace, but under constant supervision from another civilization, or choose conflict, and be obliterated–can only be fully understood against the backdrop of the Cold War-era United States, when anti-Communist hysteria was sweeping the country, stirred up by Senator Joseph McCarthy and his House Un-American Activities Committee (HUAAC). The depiction of the media’s coverage of Klaatu’s arrival and stay on Earth in the film reflected coverage of the Communist threat in the popular media at the time, as the polite, erudite Klaatu is characterized as a “monster” and a “menace” that “must be tracked down like a wild animal…and destroyed.”

Some saw the film’s peace-loving message as political propaganda, pointing to the participation of one of the actors, Sam Jaffe, who was accused of Communist sympathies and later placed on Hollywood’s infamous blacklist. In the end, the film stands up not only as an interesting milestone in the development of ufology, but as a monumental science fiction film in its own right.

Red Scare-era America also embraced the classic Oscar-winning movie The War of the Worlds, based on H.G. Wells’s novel, which took a more sinister view of invaders from another planet. A radio dramatization of the novel by Orson Welles, beginning with a series of news bulletins that suggested an actual Martian invasion was in progress, caused mass hysteria when it was broadcast on Halloween of 1938. As the 1953 film opens, the narrator intones that with their own natural resources being exhausted, the inhabitants of Mars–the Red Planet–are looking to Earth to continue their civilization.

Dr. Clayton Forrester, a famous scientist, rushes to the scene after a molten hot meteor-like object lands in the California countryside. It turns out to be an alien spacecraft, and its occupants viciously kill three men who approach the craft in friendly greeting. The military is alerted, but human weapons are powerless against the strange ships, which have begun landing all over the world.

Forrester and his love interest, Sylvia Van Buren, struggle to evade the Martians, who (in a radical departure from the human-like Klaatu) are portrayed as smallish brown creatures with three-fingered hands (to match their tripod-like ships) and a single large “electronic eye” glowing red, blue and green. Military forces around the world hit the Martians with all their firepower—even the deadly A-bomb—to no avail. In the end, the all-powerful aliens begin dying when they try to emerge from their spacecraft. As narrator puts it, they are “killed by the littlest things which God in his wisdom had put upon this earth”: bacteria.

The popularity of The War of the Worlds and The Day the Earth Stood Still, as well as that of a number of other films, including The Thing From Another World (1951), Earth Versus the Flying Saucers (1956) and Invasion of the Body Snatchers (1956) helped make the 1950s a watershed decade for ufology. One of the most high-profile UFO incidents of the decade involved George Adamski, who claimed to have met a friendly visitor from Venus in the California desert on November 20, 1952.

Adamski became a kind of hero to the budding ufology movement, but some have argued that he was less than honest, and that much of his story bears strong similarities to aspects of films such as The Day the Earth Stood Still. This was especially true of his account of the extraordinarily human-like alien, who according to Adamski radiated a “feeling of infinite understanding and kindness, with supreme humility.”

Another notable “contactee” incident came in the early 1960s, when the New Hampshire couple Betty and Barney Hill claimed to have been abducted by aliens. In the investigation of the case, the Hills’ ongoing accounts of the abduction—retrieved partially through hypnosis—were also found to bear strong parallels with various media representations of alien invasions, including the 1953 film Invaders from Mars and an episode of the science fiction anthology television program, "The Outer Limits."

By the mid-1970s, UFOs and the surrounding subculture had not lost their momentum as a popular distraction; even President Jimmy Carter, elected in 1976, claimed to have seen a UFO. In 1977, Columbia Pictures released Steven Spielberg’s Close Encounters of the Third Kind, with a massive advertising push touting the movie’s tagline: “Watch the Skies.”

Based on The UFO Experience by Dr. J. Allen Hynek, the scientific adviser to three UFO studies conducted by the U.S. Air Force, the film depicts many aspects of actual UFO incidents reported to Hynek, though many details and circumstances were manipulated for maximum dramatic effect. Set in the present day, the movie opens with the arrival of a French scientist, Lacombe, in the Mexican desert, where strange sightings and sounds have been reported to have come from the sky. The team later investigates similar occurrences in Malaysia and India, eventually piecing together a system for communicating with the UFOs and learning the coordinates of their next landing.

Meanwhile, in Indiana, electrical repairman Roy Neary meets Jillian and her young son, Barry, when all three come into contact with the same brilliant flying objects. Barry is abducted by cosmic visitors, while Jillian and Neary become obsessed with the same mysterious shape, a pyramid-like form with a flat top. When they see news reports of a mass evacuation of the area around Devil’s Tower in Wyoming–an evacuation that the Army achieved by faking reports of a poisonous gas leak–both recognize the peak as the strange shape they have been envisioning. Once they arrive, they realize a number of other people around the country have had the same vision; all of them have experienced a “close encounter.” Neary and Jillian escape the Army’s supervision and are able to witness the climactic spectacle: the first human contact made with the UFOs and their occupants.

Some conspiracy-minded ufologists viewed Close Encounters as a concerted effort masterminded by the U.S. government to introduce the public to the concept of friendly aliens. The aliens depicted in the film are decidedly more benign than any previous incarnations: child-size, with large heads and protruding bellies, they have largely featureless faces with deep-set eyes. They return their human captives, including Barry, unharmed. At the end, after Lacombe makes the hand signals he has devised to communicate, the lead alien actually seems to smile before heading back onto his ship, taking Neary back with him as an ambassador from Earth.

The success of Spielberg’s film made an immediate and international impact: when a United Nations meeting was convened in late 1977 to discuss UFOs, delegates were shown Close Encounters as a talking point. In January 1979, the British House of Lords even held a three-hour-long debate on the subject of UFOs and a motion (eventually defeated) that the British government should make public what it knew about them.

The vision of aliens as friendly, even cuddly beings was further enhanced in movies like Spielberg’s E.T.: The Extra-Terrestrial (1982) and Ron Howard’s Cocoon (1985). It was a far different vision, however, that would inform the next generation of UFO-themed movies a decade later. The biggest of these, Independence Day, arrived in July 1996 amid a frenzy of anticipation. In the movie, the scientist David Levinson joins forces with Steve Hiller, a U.S. Marine Corps pilot, to spearhead the defeat of a menacing army of aliens in flying crafts that are targeting Earth’s major cities. When the alien ships turn out to be satellites sent by a massive mother ship hovering above Earth, Hiller and Levinson are sent to plant a nuclear device on the mother ship to destroy it, while President Bill Whitmore commands an attack by U.S. fighter jets on the alien satellite ships near the classified Area 51, in Roswell, New Mexico.

A blockbuster hit dismissed by many critics as a poorly written, special effects-laden knock-off of The War of the Worlds, Independence Day marked a return of the view of aliens as hostile invaders seeking to destroy the Earth. In a moment of pointed humor, it portrays a group of giddy ufologists who gather under the alien’s ship ready to celebrate the arrival of Close Encounter-style friendly aliens, only to be massacred. Independence Day also reflected the continuing public fascination with the idea of an alien invasion, and specifically with the mystery surrounding the Roswell site in New Mexico, long believed to be the center of all the information that the government and military are hiding about UFOs. This fascination was also a central focus of the popular TV series "The X-Files" (1993-2002) and other hit movies, such as Men In Black (1997).

In 2005, Steven Spielberg–the creator of E.T., decidedly film’s cutest and friendliest alien–declared that the time was ripe for his updated version of the ultimate hostile-alien-invader movie, The War of the Worlds. The film, starring Tom Cruise, is not a faithful remake of the 1953 version, or of Wells’s novel, but its central plot line and message remain consistent—a race of intelligent, merciless extra-terrestrials are invading the Earth, and must be defeated to avoid the destruction of the human race.

The shadowy nature of the enemy in Spielberg’s War of the Worlds—the aliens are “tripods,” and not specifically Martians—suggests the changed nature of the threats facing Western society today. Compared with a nation (Nazi Germany, in the case of the Orson Welles broadcast in 1938, or the Soviet Union, in the case of the 1953 film), the lurking enemy of today–terrorism–is shadowy, evasive and indistinct. But the threat is still there–and so is the public’s fascination with the idea of UFOs and alien invaders, six decades after Kenneth Arnold’s sighting turned peoples’ eyes to the skies. If history is any guide, it is a fascination Hollywood will continue to reflect—and exploit—for years to come.