Take sides
Definition 1 of 1
Pro Tip 1/3Neutral Implication
Using 'take sides' often implies the situation is not one's personal conflict, but choosing to support one side over the other. 
Even though I was neutral, they expected me to take sides.
Pro Tip 2/3Emotional Intensity
'Take sides' can suggest emotional involvement in a dispute, indicating support is not merely logical, but also emotional. 
Sara couldn't just observe; she had to take sides because it was personal.
Pro Tip 3/3Conflict Avoidance
'Refuse to take sides' shows a deliberate avoidance of conflict, used to maintain neutrality or peace. 
To maintain his friendships, he refused to take sides.