The original film Red Dawn shows Fortress America invaded and occupied in the 1980s by the Soviet Union and its Cuban and Nicaraguan allies. The onset of World War III is in the background. The remake takes place in the modern day 2010 with North Korea invading the United States. Far fetched, maybe. But there have been been various books and films like It Happened Here, which tells of England vanquished by the Nazis and ruled by the British fascists. And then there’s the 1987 Amerika starring Kris Kristofferson, Mariel Hemingway, Sam Neill and Robert Urich. In fact, it’s a common trope:
A plot device that depicts the United States as a target of foreign occupation by another (nation-)state or states. The work will usually focus on the heroic efforts of either or both the US Armed Forces and the La Résistance as they try to defend their homeland.
August 1940: Lightning in the Night was a novel by Fred Allhoff serialized in Liberty magazine about the invasion of America. It was perhaps the first in a long series of apocalyptic porn.