This song refers to the media's reactions to terrorist attacks. It was controversially released as a single in the UK on July 9, 2007, two days after the second anniversary of the London bombings, which killed 52 commuters.
Bloc Party's frontman Kele Okereke told Billboard magazine May 2, 2006: "The 30 bus in Hackney, which is just around the corner from where I live, was blown up. [That song was] written when I was just observing the reactions of the mainstream press in [the UK] and I was just amazed at how easy it'd been to whip them up into a fury. I guess the point about the song for me is post-September 11th, the media has really traded on fear and the use of fear in controlling people."
The title is a reference to the Salem Witch Trials, which took place in Salem, Massachusetts in 1692. Women accused of practicing witchcraft were put to death, and the event continues to serve as a metaphor for overreaction and mindless judgment.