choose sides
1. To ally or agree with one person or group in an argument. I refuse to choose sides between my two daughters! I didn't intend to choose sides, but Nancy made the stronger case.
2. To divide a group of people into teams, as when playing a sport or game. The game hasn't started yet because it's taking those guys forever to choose sides!
choose (up) sides
to select from a group to be on opposing sides for a debate, fight, or game. (Fixed order.) Let's choose up sides and play basketball. The children chose up sides and began the game.