take sides against
take sides against (someone or something)
To join or form a group that rejects or is opposed to someone or something. I was really surprised when the manager took sides against Carol during the board meeting—I really thought she'd stick up for her. A coalition of 12 countries has taken sides against the aggressive military expansion of the totalitarian nation.
take sides against someone or something
to join a faction opposing someone or something; to establish a faction against someone or something. Both of them took sides against me. It wasn't fair. We took sides against the bank.