Alien Invasion deals with scenarios in which extra-terrestrial life tries to conquer or destroy the human race, usually via a direct (or hidden) assault upon the planet Earth - the idea of Alien Invasion is rife in sci-fi, horror and doomsday fiction : some conspiracy theorists fear we are in the process of a real Alien Invasion while some religious societies dismiss the idea of Alien Invasion in favor of Demonic Invasion, some blend the two together to form the controversial theory of Demonic Aliens : often linked to the idea of the Divine Delusion.
Alien Invasion has also been deemed as one of humanity's greatest threats of survival by some fringe scientists, though most mainstream scientists still put the idea squarely in the realms of science-fiction.
Methods of Invasion
the most common method of alien invasion is the appearance of massive space-ships, often equipped with devastating weaponry that can destroy entire cities - these UFOs often target major world cities and attempt to either blitz the human race into submission or simply wipe them out.
Often UFOs have a weakness that is exploited at some point by protagonists, such as the infamous computer-virus that ultimately saved the human race in the original Independence Day, prior to finding a vulnerability most UFO invasions seem to be invincible and current-day military might is hopeless against the aliens' immense fire-power and shielding.
"Tripods" / War-Machines
another famous method of invasion, made popular by the War of the Worlds, involves aliens sending gigantic war-machines to conquer or devastate the world : these machines are often "walker" type drones that are equipped with death-rays or abduction-beams designed to ruthlessly hunt down, capture or exterminate humans : they are smaller than UFOs and usually land-based, they are however more dangerous than the standard alien invader and much like UFOs it requires luck or knowledge of a weakness to finally topple these machines, which are often invulnerable to any form of resistance up until such a point (in War of the Worlds the machines were only stopped when the Martians died of the common cold, due entirely to "luck" on the part of humanity).