The "They're really not aliens" interpretation
It's possible that the aliens aren't really extraterrestrials -- as we conventionally understand 'em, at least -- but rather demons from the underworld (the dark spiritual dimension) invading the physical realm to take as many victims as they can. The idea that they are aliens is just a mass deception and, remember, the devil is the "father of lies."
There are many clues:
- The spaceships are never seen, just lights in the sky (the devil's also called "the prince of the power of the air" in the bible).
- The "aliens" look decidedly demonic and have sorta cloven hoofs.
- The primitive method of repelling the aliens is discovered in three cities in the Middle East and is holy water, which is ordinarily used by Catholic & Episcopal exorcists against demonic manifestations (Shyamalan, by the way, attended Catholic & Episcopal schools).
- The three cities in the Middle East could represent Father/Son/Holy Spirit.
- The daughter, Bo, was said to be a "gift from God" who was sensitive to impure water. People's mouths dropped when she was born as she was spoken of as an angel. Her name in Old Norse means "to live" and she was the messenger from God (an angel) able to discern the weapon of life needed to defeat the demonic menace.
- The way the "holy water" affects the "aliens" is reminiscent of the way a cross/crucifix affects vampires or a demon-possessed person in movies.
- The dog and bird were clearly possessed.
- The crop circles are occultic symbols and possible portals from the underworld.
- The crop circle in the Hess cornfield resembles a pitchfork.
- When the disillusioned minister cries out to God that he hates him he actually displays his faith; after all, he has to believe in God to have a conversation with him, voicing his ire. Suddenly his son recovers from the asthma attack and the news announcer declares that the "aliens" are retreating. You see, FAITH sends the enemy fleeing with their tails between their legs.
Also, we see evidence that the demons are personal in nature. The "alien" locked in the vet's pantry was linked to the guilt and disillusionment that haunted the vet after the death of the minister's wife.
Keep in mind that when the film debuted, Shyamalan was known as the king of the plot twist, yet the revelation that the aliens’ weakness is water wasn’t exactly a mind-blowing turn seeing as how the earth is 71% water and technologically advanced extraterrestrials who can travel the galaxy would know this. Moreover, the aliens curiously have no physical weapons and are easily trapped by a pantry door. It stands to reason that Shyamalan hid the twist in the movie. That twist is that the aliens are really demonic in nature.
I'm not saying you can't view the movie as a literal extraterrestrial invasion flick, but rather that the underlying demonic reading is a legitimate way of viewing the material because it all ties together and links to the story arc of the disillusioned ex-minister finding redemption. Also, I'm not suggesting that you have to believe in God to appreciate this movie, which is obviously not the case. I don't believe in androids traveling back in time, but that doesn't prevent me from appreciating the "Terminator" flicks.