take somebody's side
pick somebody's side , also: take the side of somebody
choose somebody's side

It will be too late then to choose your side.

I don't want to watch as my children have to choose sides.

There comes a time when one must choose sides, yes?

There comes a time when a man has to choose sides,' he told them.

Still, he had chosen his side, and now it was up to me.

The young lady said before she had chosen her side.

But it's too early to choose sides; I need to understand what is going on.

My friends had run toward where the other kids were choosing sides for the game.

You said a while ago that it was time to choose sides.

At least out here, right now, we don't have to choose sides.