Here's a proposed chronological marathon order, with explanations:
Part 1: Origins and Early Days (Mostly X-Men Focus)
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X-Men: First Class (2011)- Placement: Set in 1962 during the Cuban Missile Crisis, it's the earliest point in the X-Men film timeline, showing the origins of Xavier's School and the formation of the first X-Men team.
- Explanation: This is a clear starting point for the X-Men side, establishing Professor X, Magneto, and the early mutant world.
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Captain America: The First Avenger (2011)