I remember reading somewhere that the writers stated the aliens who invaded us in Battle: Los Angeles were supposed to be their version of a third world crap army and didn't represent the most advanced force they could field.
It also seemed to me that the aliens were not intent on actually conquering the planet, but just raiding our world. They needed water and I think they were just trying to hit a few costal cities and hold us back long enough to grab the water they needed. This is evidenced by the numbers they deployed to Earth (which is never stated in the movie, but it was stated on the official website for the movie as part of their marketing campaign). The aliens only sent about 30 million at most, which is not enough to subjugate a planet of over 7 billion (especially since their tech advantage over us was minimal).
I think, given all of that, Battle: Los Angeles is a fresh take on the alien invasion scenario. It shows Earth being invaded by a rag-tag and possibly desperate alien force that just seems to be interested in conducting Viking-style raids rather than the enslavement, extermination, and conquest other sci-fi aliens seem to be interested in. It also shatters the idea that aliens are always superior to us and shows that aliens can be just as dysfunctional, chaotic, and unstable as we are. I think after reading the backstory that was never touched upon in the movie, the writers were also trying to warn us that we may end up like those aliens if we continue down our current path.
Here is the link to the site that gives some of that backstory I'm talking about:
Link