As Swift said in her Rolling Stone interview:

For years, I was never sure if we were friends or not. She would come up to me at awards shows and say something and walk away, and I would think, ‘Are we friends, or did she just give me the harshest insult of my life?’ Then last year, the other star crossed a line.

She did something so horrible. I was like, ‘Oh, we’re just straight-up enemies.’ And it wasn’t even about a guy! It had to do with business. She basically tried to sabotage an entire arena tour. She tried to hire a bunch of people out from under me. And I’m surprisingly non-confrontational – you would not believe how much I hate conflict. So now I have to avoid her. It’s awkward, and I don’t like it.

Much speculation has been made about who this song is about, with one popular theory suggesting Katy Perry. Perry seemed to think so herself, tweeting the day after the Rolling Stone feature was published: