Sentinels, robots that were created for the purpose of hunting down mutants were released in 1973. 50 years later the Sentinels would also hunt humans who aid mutants. Charles Xavier and his X-Men try their best to deal with the Sentinels but they are able to adapt and deal with all mutant abilities. Charles decides to go back in time and change things. He asks Kitty Pryde who can send a person’s consciousness into the person’s past to send him but she can only send someone back a few weeks because if she sends someone back further it could harm them. So Logan decides to go back himself because he might be able to withstand it. So Charles tells him that it’s Mystique who’s responsible because when she learned about the Sentinels she sought out Bolivar Trask the man who created them and killed him.