Taylor Swift believes Katy Perry is a thief ... and we've learned that's the basis of their feud.
The girl-on-girl war ignited when 3 of Taylor's dancers jumped ship and joined Katy's tour. Taylor felt betrayed because she became very close to the dancers and considered them family.
But here's the thing. The 3 dancers had actually worked with Katy first, but when Katy's tour ended several years ago they needed work. So they signed up as dancers during Taylor's tour.
We're told the dancers told Taylor up front ... they did not want to sign for the full tour -- they wanted a 30-day out, and we're told it's because they were just waiting for Katy to go back on tour.
When Katy hit the stage again, they gave their 30-day notice, but Taylor got so pissed off she fired them on the spot.
All of this went down a year ago but we're told Taylor held a grudge, which boiled over when she wrote a song -- "Bad Blood" -- alluding to Katy screwing her over.
It's turned into a full on war, with Taylor not-so-coyly talking about the alleged betrayal -- and Katy responding on Twitter by calling Swift ... Regina George in sheep's clothing.