"It is important for a general to be calm and remote, upright and disciplined." Sun Tzu
What works is clear thinking. That is what people are good at. But many people remain convinced via their actions that emotions should rule them. They move here and there depending on how they feel. This is dangerous because they are easily manipulated by others who use this weakness to their advantage.
You achieve calm by first realizing it isn't a power. It is a choice. You choose to be calm, even in the midst of chaos.
Think about the times when you were in a scuffle or were involved in one. Did you act like you wanted to act that day? Or did you forget how you should conduct yourself and act on impulse?
In other words, being calm is about remembering to employ your true strength: your reasoning ability. You choose to remain calm and therefore achieve power over yourself, and in turn, power over others. They are no longer enemies under Heaven as they can do no harm to you.