Re: So what will be this Preventable Event?
People are Ignoring the obvious possibilty and how they tie the films together.
Post First Class Magneto could plot assassination.Since Matthew Vaughn wrote the treatment and If they really want to go down the alternate future path It could be JFK.
The assassination Is world's public exposure to mutants.Thus beguins the antimutant hysteria.The government started the weapon X Project to use mutants as weapons
against other mutants.Eventully the registeration act Is purposed.This leads to events of trilogy.Post trilogy antimutants devolp a new weapon The Sentinles.They are used to
try to deal with the so called Mutant problems.The sentinles turn on creators.
You've just connected all the films there.