The phrase "to pit sth/smb against sth/smb" means to force them into a fight/debate/argument/competition Examples: "Every election in America just seems to be the democrats and republicans pitted against eachother." "Spider-Man was pitted against Venom after Eddie took the symbiote." "This football match sees the Steelers pitted against the Patriots."

As I understand it, 'Pitted' can be used when there is some added intrigue associated to two opposing sides going against one another. "The quarter final draw for the Champions League has Cristiano Ronaldo pitted against his former club."