Since World War 2, the Americans have made many films and TV series about alien invasion. This must reflect some subconscious national fear.
Is it the influx of immigrants bringing new cultures, language and change? Is it conflicting doctrines and political ideas? Or is it something more obvious?
American treats war as a business. Movies speak of "American Interests" overseas. They have a massive standing military force and are perhaps technologically superior to any other military force on the planet.
My point is this: do these alien invasion stories reflect the fear that there may be an even greater military force out there? That America may face a foe that even they can't handle?