No backstory for justifying alien activities on Earth in XCom

[Kunal_R.One]

I have played XCom for long time now. One thing about the game has always kept me in doubt. Why the aliens are invading the Earth. I mean there is no backstory in the game to justify their actions. Did humans go on their planet and created trouble or the aliens want to destroy earth? It is always shown like Aliens coming in different parts of our planet and starting an insane rampage of human killing.

[Wyl]

If you see the enemy units, especially the functions of their buildings, the backstory or the purpose of alien attacks can be discovered. At one point Alien Harvest research is shown, this means Humans are being harvested by the aliens. Alien Reproduction center shows that the aliens reproduce themselves artificially. So maybe they want to harvest humans for the knowledge of natural reproduction of life form. At one stage you see slaughtered cattle, this shows that aliens are surviving on earth by consuming other life forms except humans.

[LotusN]

At some point I got the feeling that the Aliens wants to destroy the stronger part of Humanity so that they can enslave the remaining weaker humans for their purpose. At one point, they try to poison the air and water which is extremely essential for human survival. The poison actually mutates the human beings. So another objective of the alien race must ne to mutate Human beings to become one of them. This will help them colonize earth or maybe take over the earth.

[Enos]

I found a message in game and I know most of you have must have reached that point where the brain speaks,
Many millions of years ago the planet you call Mars was alive. This life was brought to a barren planet by our civilization as it was to yours. For millions of years we have visited your planet and genetically developed your species. You cannot kill us; you are a part of us...
Here is the centre of Martian civilization - the pyramids built millions of years before yours - by a species which is your ancestor. No planet is beyond our reach. This power could be yours before long. All we ask is your co-operation...
[Kills brain]
Once the aliens lost Mars they have lost the earth. Before long XCom research allows humanity to flourish once more, and claims Mars for itself. The alien menace has gone, but for how long, no one knows...
I think this whole dialogue must have some meaning, I couldn’t figure out much.

[Orochimaru]

The first line suggests that, aliens that are now attacking the earth had themselves bought life on earth. They themselves have been visiting our planet since long time. They are the one who engineered and helped humans evolve. The pyramids show that once humans achieved the skills of learning, the aliens trained us to build structures that can be lived in. This message also shows that the aliens have power that is beyond reach of human beings. But then the aliens lost the planet Mars as well s Earth and now they are back to take what was once theirs.