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The back story is basically that huge alien ships called arks showed up and started crashing towards earth. Some landed, bringing several alien species to Earth. Humans and aliens need to learn how to coexist while they are both under the threat of yet another hostile alien civilaztion that wants to terraform the earth and kill the rest.
Defiance is actually the name of St. Louis which is a safe haven (protected by an energy field?).
This is my own summary based on watching interviews and trailers for both the game and the TV show.
The alien technology plays a central role in the game and show as it allows both humans and aliens to become far more powerful.
You do realise that this is a tie in with a Sci-Fi series don't you ?