Observing rather than taking part, out of the action, as in Bolivia's neighbors remained on the sidelines, waiting to see which faction in the dispute would prevail. This idiom comes from sports.

What does to stay on the sidelines mean?

If you stay on the sidelines, you are not an important part of what is happening.

Is on the sidelines an idiom?

If you are on the sidelines in a situation, you watch it happening but are not directly involved and cannot influence it. France no longer wants to be left on the sidelines when critical decisions are taken. The government has just stood on the sidelines up to now and let the situation get worse.