The storyline focuses on present-day LA, and rather than the 1942 false alarm being caused by jittery war nerves, the anti-aircraft guns were actually shooting at an extraterrestrial "scout" spacecraft spying on southern California nearly 70 years before the aliens landed.
Movies featuring angry aliens are always fun (unless you had to sit through Spielberg's "War of the Worlds," that is), but ignoring our obvious uneasiness with the phantom alien menace that might lurk in our galaxy, there is one question that keeps popping up in my mind: Why would extraterrestrials attack us?
Resources? A Strong Dislike for Humans?
You could counter that question with: Why wouldn't they attack us? but either way, the answer is as speculative as the search for extraterrestrial life itself. There's no precedent for life beyond Earth, we only have Earth Brand™ Life to work with. And no, I don't believe for a second that they have visited Earth already.
So, looking back at our own history, the root causes of most conflicts are resources and ideology. If one country has a resource another nation wants, they might take it, with force. If one nation has a different ideology to another, they might attack that perceived threat.