I think you are required to join it, but proceeding to actual Euro rollout requires a currency to be within a certain range for a period of time, and that requi

I think you are required to join it, but proceeding to actual Euro rollout requires a currency to be within a certain range for a period of time, and that requires active intervention in currency markets. Governments can just either not do it or claim they can't intervene enough to stay on target.