Alien Invasion

Now this is the first thing almost everyone thinks of when they hear the words: Science Fiction.

The first recorded story of an alien invasion is War of the Worlds, in which Earth is being attacked by octopus-like creatures from Mars.
Ever since then, there have been many other tales of beings from another world trying to do harm to earthlings. The early stories, the villains were mainly Martians. That is because ever since the supposed discovery of "canals" by Lowell, people became fascinated by the Red Planet and the possibility of life existing there. But now, since we have sent rovers to Mars and have not found any relevant signs of life, those storied have died out a bit.

The aliens that come to our planet are usually much more advanced than us. After all, they have to be in order to to travel such great distances in space. Sometimes they are here to help us or save us from ourselves, such as in the film, The Day the Earth Stood Still. But many times, the intruders are warlike beings who will stop at nothing to annihilate the human race. Usually, the reason is that is that their own planet is in danger, or they need resources, or they just want to take over. Whatever the reason, The aliens would blow up everything in sight with their fleet of UFOs.
There are some stories that are different however. Like the movie Evolution in which Asteroids containing bacteria land on earth, and they start to reproduce and evolve at an incredibly high rate. Many stories don't even involve large fleets of space ships and death beams laying waste where cities once stood. Sometimes the extraterrestrials just work quietly under cover, or disguise as humans and even get jobs on Earth. There is a great show called the X-files, involving government cover-ups and such. In the movie Signs, The aliens use hand to hand combat, and things other than mass destruction, for if their do, they will cause humans to use Nuclear weapons, and the planet will be worthless for them. In some movies The human travel to the extraterrestrial plant and bring the war to THEM.
No matter how powerful the aliens are we always find a way to beat them, weather we get lucky, or we discover a weakness in them.

Recommended films:
War of the Worlds(1953)
War of the Worlds(2005)
Independence Day
Star ship Troopers
The Day the Earth Stood Still
X-files
Earth vs the flying saucers
Mars Attacks
Signs
Evolution