The movie INDEPENDENCE DAY: RESURGENCE, about a military-style alien invasion of Earth, opens today.
Of course, nobody knows what an alien bid to take over Earth would look like, but a direct attack is perhaps least likely. If a civilization capable of interstellar travel wanted Earth for some reason, it could simply direct a big asteroid to collide with our world and wait. Or, if They wanted Earth intact, introducing a virus or two to wipe out humanity would do the trick. Risking combat would not be necessary, but it makes the best gee-whiz action movie.