Alien invasions in Hollywood follow a pretty simple format -- the initial strike is unexpected and catastrophic, leaving civilization crushed like an ant mound under a jackboot. This is done just to show the audience the stakes. And then comes two hours of character development, crying and confused masses running.
If you're lucky, Will Smith might shove his fist into a mass of tentacles.
Independence Day in particular relies heavily on a chess analogy, which Jeff Goldblum bluntly points out for those who can't be bothered to make the connection with all those seemingly unrelated scenes of chess games scattered throughout the film. The aliens make their move, then wait for ours. Although this is mighty polite and shows that aliens have a highly developed sense of warfare etiquette, there's no reason to allow your inferior human opponents to develop a defense strategy after it's clear that you have them by the balls.
The whole reason Goldblum has time to come up with his eventual war-winning strategy is that the huge, city-destroying saucers circumnavigate the globe at a pace so slow that it takes them days to destroy the world's cities -- they make their trip from Chicago to Houston slower than a sedan running on four flat tires. In Skyline, the aliens get out of their ships and run around naked (sigh), picking off humans individually, even though in the opening scene it is very obvious they have some kind of giant tractor beam that can just suck all the humans out of the city in 10 minutes.
Congratulations. You have now seen everything worthwhile about Skyline.
Here on Earth, outside of video games, turn-based combat has been out of vogue since the days of pistol duels. While it may occasionally be worthwhile to hang back and see what your opponent is capable of, if you've taken our advice so far and done your research, you'll note that our experience in space combat is limited to our conquest of Earth's moon about 40 years ago, and that wasn't much of a battle. When you have humanity by the short hairs already, there's no reason to wait for Jeff Goldblum to reverse-engineer your spacecraft and come after you unless you're in it for the thrill more than the victory.
Or you're a huge, huge fan of Randy Quaid.
So the human beings have figured out your ridiculous weakness at the eleventh hour, and you're watching in horror while your enormous, dark-gray warships go down in flames at the onslaught of these talking meat creatures. You're going to lose this battle, but why call off the war?
"We've got plenty more baseball bats and Phoenixes where that came from!"
It's the same scene we see at the end of Independence Day, War of the Worlds, The Day the Earth Stood Still and even Mars Attacks! -- civilization in ruins, our cities in flames and crushed under the massive debris of fallen warships and dead, rotting aliens. Sure, the humans won, in the sense that they're not all dead, at least not right away. But what Will Smith is really celebrating as he sassily puffs on his cigar is a big fat case of Pyrrhic victory.
The term is named after Pyrrhus, king of Epirus, who "won" a couple of battles against the Romans around 280 B.C., but his army was so decimated that he quipped, "Another such victory and I come back to Epirus alone." In other words, tiny Epirus had lost so much of its military that there was nothing stopping the Roman Empire from walking in and taking power, which it of course did.
But hey, Pyrrhus got a whole phrase. And gorgeous bangs.
Likewise, human civilization is in a sorry state after an alien invasion, and not just because it's in deep mourning for the loss of its landmarks. Millions, maybe billions, are already dead, in many cases the U.S. government is gone, and with the cities and huge chunks of infrastructure destroyed, there's going to be a huge problem with the food supply. And that's before the disease sets in from all those dead people and aliens rotting in the sun. In short, after our victory against the alien hordes, we're looking at a worldwide Mad Max situation.
But what have the aliens lost? An investment? The attack took place on our world, not theirs. Their home is still perfectly intact. They get to retire to their comfortable, presumably nondystopian home world and muse about our bad luck. We don't know anything about the economic impact of financing an intergalactic attack campaign, but if you wanted it bad enough the first time, there's no reason not to go for Round 2, especially since you're now aware of the bullshit weakness that thwarted you the first time. Consider throwing on a hazmat suit the next time.
Maybe splurge on some antivirus software.
Of course, before you even begin thinking about how to best organize a strike against Earth, you need to ask yourself whether it's worth going to that kind of trouble.
We don't always know why the aliens want to kill us. In some cases, the question remains open. Sometimes they just enjoy it, like in The Day the Earth Stood Still, where the aliens are left-wing extremists out to stop global warming. In Skyline, they needed our brains for some reason. But in most cases, they just seem to want our resources. In Independence Day and the upcoming Battle: Los Angeles, the stated goal of alien invasion is resource acquisition -- they want our shit.
Thing is, if you have the technology for space travel, space mining and space ExxonMobil, there's really no reason to attack Earth for the same resources that you'll find on Mars. In fact, this solar system alone has eigh- seven planets (sorry, Pluto) that won't shoot at you, and they're all made of the same stuff. In fact, you can even mine the moon, and there isn't a whole lot that the landlocked denizens of Earth can do to stop you.
Come on, ISS. Bring it.
But just say that nothing short of Earth's unique ecological profile will meet your needs. Maybe you want water (unless you're from Signs), and we admit that's pretty rare in the universe. But maybe you should take a look at one of the universe's many "Goldilocks planets" (so named because they're not too hot and not too cold). Statistically speaking, this galaxy alone should be littered with planets just the right size and distance from their suns to develop liquid water, an Earth-like atmosphere and all the raw minerals you need. And either all of them or most of them are devoid of the kind of life that can shoot missiles at you and hack into your mother ship.
These movies are usually trying to parallel the reasons we fight wars here on Earth, and while we do fight for resources, we have the good sense to secure uncontested minerals before we consider waging an expensive and violent war. You can bet mankind will be mining Mars before we consider sending warships to Zeta Reticuli.
THERE ARE BETTER WAYS TO MINE.
Really, guys, this stuff is just common sense.
You aliens should really pick up our new book. It'll serve you right when you're attempting to demolish humanity.