The easiest way to write an Alien Invasion is to make the invaders nigh unstoppable by giving them technology and weapons we can't even comprehend, defend against or overpower. If they were just like a human army, it might as well be a war story; there must be something about the aliens that makes them distinct from any other army. But the heroes and humanity can't just be dispassionately crushed under a cosmic steamroller, because a Cosmic Horror Story would be a box office dud due to the bleak narrative and Downer Ending. Besides, who would want to read that kind of story?