Lesson one: The aliens never, ever come in peace

Global warming and the occasional hurricane aside, the Earth is a pretty nice place to live. No wonder aliens are constantly trying to conquer it. The latest assault by hostile visitors from outer space comes in "Battle: Los Angeles," debuting in theaters March 11.

If it seems a little familiar, well, it is — the basic template of alien-invasion stories has been in place for more than 100 years, ever since novelist H.G. Wells created the definitive model in 1898's "War of the Worlds." Here's a look at the time-honored traditions — or, when done badly, the hoary cliches — you'll find in nearly every alien-invasion movie.

1. More Entertainment stories
   1. Autistic ballerina dances her way into hearts
      

      In a popular YouTube video, the beaming little ballerina dances an entire four-minute routine seemingly perfectly, matchin...

   2. Every on-screen drink in 'Mad Men' in 5 minutes
   3. See the 'Dancing' stars' most memorable moves
   4. Emmy's biggest snubs? Cranston, Hamm, more
   5. 'Toy Story' toys burn up in prank on mom

The invasion is either overwhelming or completely secret
Wells' original tale set the standard: Martians come to Earth in massive numbers and with technology so advanced that humanity seems surely doomed. There's no subtlety here: We are bugs, and they are a very large shoe. It's hard to improve on that simple, brutal premise, and it's no surprise that it returns again and again in movies like "Independence Day" and "Skyline." "Battle: Los Angeles" falls squarely into this mode — the aliens arrive in a Pearl Harbor-like hail of exploding meteors, and come out shooting.

On the flip side, there's the strategy seen in "Invasion of the Body Snatchers," "The Thing," and "They Live": The aliens go undercover, and are so good at infiltrating us that we don't even know they're here. There's not much middle ground, although the TV series "V" splits the difference — its massive alien battle fleet pretends to be friendly, but secretly takes over world government the way the Nazis occupied France.

1. More Entertainment stories
   1. Autistic ballerina dances her way into hearts
      

      In a popular YouTube video, the beaming little ballerina dances an entire four-minute routine seemingly perfectly, matchin...

   2. Every on-screen drink in 'Mad Men' in 5 minutes
   3. See the 'Dancing' stars' most memorable moves
   4. Emmy's biggest snubs? Cranston, Hamm, more
   5. 'Toy Story' toys burn up in prank on mom

Attack of the metaphor from outer space
In the 1950s, aliens were often a stand-in for Communists, a reflection of the paranoia of the Cold War era — apparently, there's a reason Mars is red. But the best science fiction is often allegorical, using the outlandish concepts of the genre to cast new light on problems we've already got here on Earth. The original 1980s version of "V" is great example of this, with its alien invasion specifically conceived by writer Kenneth Johnson to parallel the rise of the Nazis with something similar happening to America. Wells had a similar goal, using Martians to satirically point out how many clashes between Earth cultures have ended in genocide. And "District 9" and "Alien Nation" both used the notion of "invasion" by pathetic alien refugees to make a point about prejudice.

No, we do not come in peace
What's the quickest way to show that the villains are really, really mean? Have 'em kill somebody who was only trying to be friendly. What this means to the story depends on the attitude of the filmmakers. In George Pal's version of "War of the Worlds," it's noble martyrdom: A peace-loving priest who consciously walks into the line of fire just so humanity can say that they at least tried to be friendly. In "Independence Day," it's a bunch of woolly-minded hippies who have no clue they're partying beneath a gigantic laser cannon. That's just a Darwinian culling of dumb people from the gene pool. Either way, it establishes that the aliens aren't here to welcome us in galactic harmony.

Cosmic Log: Why we love to fear E.T.

The next item on the invasion agenda is to blow up something famous. The 1953 movie version of "War of the Worlds" breaks the Eiffel Tower in half, and 1956's "Earth Vs. The Flying Saucers" topples the Washington Monument and shatters the U.S. Capitol dome. "Independence Day" offers the biggest orgy of destruction, as the massive motherships simultaneously demolish the Empire State Building, the White House, and dozens of major cities in one fell swoop. It's like some twisted parody of vacation snapshots at tourist traps, which the "Mars Attacks" villains point out succinctly by getting a photo op at the Taj Mahal while they're burning it down.

One big speech stirs up humanity's will to fight back
Winston Churchill knew well how powerful a good speech could be to restore people's courage in the face of an overwhelming threat — "we will fight them on the beaches," and all that. It's no surprise that similar moments crop up time and again in alien-invasion stories. One of the most memorable came in "Independence Day," when president Bill Pullman rallied his troops for a counterattack by declaring that after the battle, the entire world would celebrate July 4. Cheesy and sentimental patriotism? Sure, but at the same time, kind of awesome. Ironically, H.G. Wells cut this cliche off at the knees before anyone even did the straight version: In the novel, the narrator encounters a delusional survivor who rants of big plans for resistance, to fight the aliens from tunnels, steal their ships, and take over the world himself — never mind that he's about as threatening to the Martians as a yappy little terrier.

1. Most popular

Excuse me, you forgot your one fatal weakness
Aliens have all kinds of impressive technology to conquer us with: disintegrators, force fields, and spaceships that can cross the interstellar void. But they don't have flu shots. And that one forgotten fact saves the day. This is another element that's as old as the genre: H.G. Wells' Martians had humanity outmatched but underestimated the lowly virus, and all died of disease on the verge of their final victory. "Independence Day" gave that concept a clever update by having the humans knock out the mothership with a computer virus — clever, except that you had to swallow the ridiculous idea that the virus was something Jeff Goldblum could whip up on his Macbook.

For all the brilliance of Wells' twist ending, it's all too easy a way for weaker storytellers to escape once they've painted themselves into a corner. Satirists have sent it up repeatedly: In "Mars Attacks," Martian craniums explode when they hear the plaintive yodel of Slim Whitman's "Indian Love Call." And in Douglas Adams' "Hitch-Hiker's Guide To The Galaxy," an invasion is thwarted when "due to a terrible miscalculation of scale, the entire battle fleet was accidentally swallowed by a small dog."

But the hands-down dumbest alien Achilles heel has to be in "Signs": Water burns them like acid. Hey, did you guys miss the giant oceans covering 70 percent of our planet? Next time you invade Earth, bring your galoshes.

Christopher Bahn lives in Minneapolis and is a regular contributor to TODAY.com.