Martians (War of the Worlds)

"No one would have believed, in the last years of the nineteenth century, that human affairs were being watched from the timeless worlds of space. No one could have dreamed we were being scrutinized, as someone with a microscope studies creatures that swarm and multiply in a drop of water. Few men even considered the possibility of life on other planets and yet, across the gulf of space, minds immeasurably superior to ours regarded this Earth with envious eyes, and slowly and surely, they drew their plans against us."

- H. G. Wells on The War of the Worlds

The Martians are the chief antagonists of The War Of The Worlds book and the varied books, musicals and other works based on it - originally an unstoppable force that almost succeeded in wiping out humanity they are famous for being defeated by the common cold, a virus that while relatively harmless to humans seems to be fatal to the Martians.

Origins
The Martians originate from the planet Mars, a planet long rumored to host life but dismissed by scientists- as they said "the chances of anything coming from Mars are a million to one", however the scientists were wrong and Mars was indeed populated -by alien beings that looked upon their closest neighbor (Earth) with envy and began to plot against the creatures that inhabited it.

Reason For The Invasion
Although it is never specifically stated why the Martians attacked Earth in the story, it is suggested that they were envious of humanity and the Earth itself, which was full of life - in stark contrast to the barren wasteland that is Mars. Also, due to their highly-advanced intelligence and warlike nature it is possible the Martians deemed humanity unworthy of the planet and decided to take it over for themselves.

Another reason for the invasion, which is explored more in the book than the original film, is that the Martians were also vampiric in nature and were harvesting humans for their blood (this theme was somewhat explored in the most recent version of the movie).

Third reason for their invasion is speculated that Mars was becoming no longer habitable to Martians so they had to migrate to the nearby planet very fast.

Personality
Ruthless and beyond reason, the Martians were depicted as much a force of nature as alien invaders and had obviously been preparing their assault for years as they easily decimated the nations of the world and were virtually unstoppable until they became ill- the Martians hid most of the time inside their fearsome war-machines and didn't communicate with humanity outside of merciless killing and thus they were probably xenophobic or simply so advanced they viewed humans as unworthy of communicating with.

They were, however, extremely intelligent and masters of war and seemed to have a masterplan on a greater scale than they ultimately achieved - it is possible that, had they not caught the common cold, they could've wiped humanity out and begun a search for other worlds to take over.

Impact On Other Works
The War Of The Worlds is considered a classic of science-fiction and the Martians remain one of the most popular alien invaders in fiction - although the popularity of Martians has begun to decline somewhat since the craze of X-Files style Grey aliens took over, however the Martians still enjoy considerable popularity - especially in Japan.

The story of War Of The Worlds and the Martians in general are also inspirations for innumerable stories and cartoons as well as parodies - some famous examples being the series Tripods and the Looney Tune villain Marvin The Martian.

They have also appeared in other media, such as The League of Extraordinary Gentlemen wherein they were the antagonists of Volume 2. Their role in Volume 2 is largely the same as the novel as the Martians ravage England and kill numerous people. Although in this incarnation, they are aided by Hawley Griffin who betrays the British Government (and the human race) so that he could rule alongside them. Ultimately, the Martians are stopped and killed by the release of a hybrid disease created by Dr. Moreau, although this results in countless casualties of human life as well. A giant statue of a Tripod is erected as a memorial of the invasion and, on a darkly humorous note, one of the cylinders is converted into a children's playground that they are too afraid of to use.

The AT-ST and AT-AT Walkers from Star Wars are obviously an homage to the tripods from the novel.