Movies let us experience a wide array of feelings and situations vicariously. In doing so, they let us deal with our own fears and anxieties in a safe, non-threatening setting. That's a large part of their appeal, and that helps explain the popularity of genre films: horror and science fiction, for example.
The United States has not been invaded militarily since the War of 1812, but during the 1950s Hollywood turned out a horde of invasion scenarios involving giant creatures or extraterrestrials. The creature incursions, whether singular ("The Deadly Mantis," "The Beast from 20,000 Fathoms") or en masse ("The Beginning of the End," "Them!"), almost always resulted, directly or indirectly, from nuclear bomb tests or experiments with radiation — at a time when people worried on a daily basis about fallout and the possibilities of nuclear war.
The hostile ETs (in "Earth vs. the Flying Saucers" or "Invasion of the Body Snatchers," say) stood in for the Soviet Union, our real-life adversary in the Cold War (and a very possible source of radioactive doom).
Then for two or three decades, things changed and movie invasions slacked off. For every "Giant Spider Invasion" there would be two or three films like "Kingdom of the Spiders" or "Day of the Animals" where normal creatures started attacking humans because somebody tampered with nature (mostly dumping toxic waste), though they usually did not leave their normal hunting grounds to do that.
Unsuspecting victims wandered into the haunts of human monsters in films such as "The Texas Chainsaw Massacre" and "The Hills Have Eyes." And with most zombie films, much horror derives from the zombies infesting the formerly normal environs where they led their conventional pre-zombie lives.
About the only memorable invasion film during the 1980s was John Milius' "Red Dawn" (1984) in which American teenagers resisted Soviet occupation forces. And in 1996's "Independence Day" marauding ETs once again made the mistake of messing with us.
But now in the 21st century, invasion films have returned in force. "Red Dawn" itself was remade in 2009 and eventually released in 2012 after its originally Chinese invaders had been redubbed as North Koreans in order not to alienate the large Chinese market. It's the only recent major release where the invaders have a direct real-world relevance, especially now that North Korea has been doing some nuclear saber-rattling.
Hollywood appears more comfortable with science fiction invasions. Unlike the 1950s films, many modern invasion films have big budgets, elaborate, fairly convincing special effects and personnel with impressive lists of credits.
Just consider Steven Spielberg's remake of "War of the Worlds" starring Tom Cruise, "Cowboys and Aliens" with Harrison Ford and Daniel Craig, "Battle Los Angeles" starring Aaron Eckhart, "Battleship" with Liam Neeson in a supporting role, and the DreamWorks animated feature "Monsters vs. Aliens" with voices supplied by Reese Witherspoon, Seth Rogen, Hugh Laurie and Kiefer Sutherland.
Many recent big summer blockbusters have been invasion movies. All the Transformers films have involved bad giant robots trying to take over the Earth. The superheroes in "The Avengers" repulse a massive attack by aliens in giant war machines. "The Twilight Saga: Breaking Dawn, Part 2" climaxes with an invasion of the Pacific Northwest by a European vampire clan (a sort of vampire "Red Dawn"?). Christopher Nolan's "The Dark Knight Rises" turns the anarchistic assaults of his first two Batman films into a full-scale invasion and occupation of Gotham City.
"Jack the Giant Slayer," now in theatres, turns the familiar fairy tale's Jack versus one giant conflict into an all-out invasion of Jack's homeland by a throng of giants. And in July, Guillermo del Toro's "Pacific Rim" will pit human-piloted giant robots against giant monsters emerging from the depths of the ocean.
Aliens also invade in lower profile theatrical releases such as "The Darkest Hour" and "Skyline." And that's not even counting all the low-budget rip-offs of the blockbusters with similar titles and plots that infest video rental shelves and less-than-premium cable channels.
Of course, we have had horrific acts of terrorism in this country, most notably the 9/11 attacks, but none were genuine invasions. And in any case, Hollywood regularly depicts terrorist strikes in a straightforward manner — that is, they don't turn the terrorists into extraterrestrials or other fanciful creatures.
"Olympus Has Fallen," for example, depicts North Korean terrorists seizing the White House and the President in an assault that recalls some 9/11 elements.
America has engaged in more than one war in recent years but not against foes like the Soviet Union with the resources to mount a full-scale invasion. So what sort of fears might the current crop of invasion films be playing to or on?
Well, even though the scope and the aims of terrorist attacks and invasions differ, invasion films surely play on the sense of vulnerability which real world terrorist acts produce.
If you look back over the titles I've mentioned, you'll notice that only the marginal "Twilight Saga" example involves an invasion which does not come as a complete surprise to those attacked. It's terrorism's unpredictability that helps make it a potent psychological weapon.
Most films I've mentioned involve assaults from the sky — even "Jack the Giant Slayer." That suggests not only unpredictability but also something over which one has no control, and there's a lot of that going around in the real world today, from the economy to climate change.
Of course, all those rampaging extraterrestrials are the ultimate illegal, undocumented aliens. It's a good bet most Hollywood filmmakers whose names appear in opening credits publicly support immigration reform.
But with so many politicians, commentators and news sources exacerbating general concerns about "illegal aliens," it's only natural — or only good business — for studios to pander, however metaphorically, to such anxieties.
None of those issues or anxieties are likely to be resolved soon, so invasion movies will continue to maintain a strong presence at the box office.
Richard J. Leskosky taught media and cinema studies at the University of Illinois and has reviewed films for more than 30 years. He can be contacted at firstname.lastname@example.org.