Alien invasion is one of the most popular tropes in science fiction and popular culture. Typically in alien invasion fiction humanity is pitched against a technologically advanced enemy threatening, normally, Earth.
Alien invasion has often been used to express social, and geopolitical concerns through allegory. In addition, the aliens can represent a threat that is difficult or politically sensitive to address. Aliens can be easily accepted as an absolute evil, or villain, as opposed to human enemies, which by their nature, must be nuanced. For example, many of the science fiction of the 1950s reflected insecurities about the Cold War and the Communist menace. Robert Heinlein and the threat of the Bugs in Starship Troopers were inspired by the command economy of communism, and the lack of individual freedom.
In video games the threat of alien invasion has been frequently revisited. The classic video game Space Invaders brought the genre to the medium. In recent years the massively popular Halo franchise centers around the story of the Human-Covenant War in the future, which, in part, culminates in the Covenant invasion of Earth in Halo 2.
The Resistance franchise portrays a different sort of alien invasion, where the aliens have conquered Earth. The game takes the player through the subsequent human resistance. This genre appeals to the "underdog" mentality. Humans are perceived as a outmatched opposition, which through the player's actions, succeed against the extraterrestrial threat.