This post contains some spoilers.
The X-Men film series, about a class of mutant superheroes and villains, has always been about the persecution of minorities. The first installment, 2000's X-Men, drew soft parallels between the US government hunting for mutants and past Nazi atrocities. The story for 2011's X-Men: First Class, was directly influenced by the civil rights movement and the contrast between Malcolm X and Martin Luther King, Jr.
In the latest installment, X-Men: Days of Future Past (directed by Bryan Singer, and starring Hugh Jackman, James McAvoy, Michael Fassbender, Jennifer Lawrence, Peter Dinklage, and Ellen Page), this theme continues with a storyline surrounding a fictional program from the early 1970s, approved by President Richard Nixon, that involves sending killer robots after America's closeted mutant population. The film's political backdrop also features a mutant-ized take on the 1973 Paris Peace Accords, where the US agreed to end direct military involvement in the Vietnam War. On top of that, the new X-Men flick deepens the alternate history explored in the franchise by suggesting that this handsome devil was secretly a mutant:
Yep. That's John F. Kennedy, the 35th president of the United States.
In the film, Erik Lehnsherr, a.k.a. Magneto, (played by Fassbender) is being held in a secret prison beneath the Pentagon for his alleged involvement in the JFK assassination. After his fellow X-Men break him out of prison, he claims that he was wrongly convicted and that he was actually trying to save the president from Lee Harvey Oswald's bullet. His reason? "He was one of us," Magneto says.
To be fair, this is a brief moment in the movie, and it is never clarified if "one of us" means JFK was a mutant or something along the lines of a friend of the oppressed mutant community. One plausible interpretation of Magneto's claim here is that it pays homage to the aborted "Princess Diana is a zombie mutant superhero!" plot from the X-Statix spin-off.
Now here's a fake mini-documentary released in November that summarizes Magneto's alleged involvement in the JFK assassination: