Age of Apocalypse

The Age of Apocalypse is an alternate timeline created by Charles Xavier's death in 1959

History
Time travelers Fitzroy and Nimrod cause an explosion that kills Xavier back when he was at college.

Without Xavier's inspiration of pacific co-existence between humans and mutants, in present day Earth has become a battlefield. Magneto formed the X-Men against the Avengers who are tasked with finding and hunting them down.

This causes a future where the Sentinels have totally taken over without remaining opposition.

In the comics
Besides Xavier's death in the past, Apocalypse awakens earlier than in the main timeline and takes over the United States (hence the name "Age of Apocalypse). Magneto forms the X-Men to honor Charles' memory.

Instead of "human vs mutant", the conflict is "opressive mutants vs peace-seeking mutants".