The process in which beings from beyond Earth of higher intelligence then Humans invade and conquer our planet. In Hollywood, most films depict a human victory against the aliens, but War of the Worlds, directed by Steven Spielberg, has the aliens die of germs, which is far more likely. However, most scientists think that aliens would completely defeat humans, in a more of "extermination" than a war. Alien invasion is taken seriously from the UN, to the UK, to the USA.
The Pentagon has a plan for what to do in the case of alien invasion.