It is very possible for one to be calm in all circumstances regardless of whether they are designed to cause pain or pleasure. To the extent that one can control his attitude, then he can be happy in all situations, whether good or bad. The circumstances that make one to be happy come from the outside. Hence, if one is able to be detached to the outside circumstances and not be controlled by them, then happiness will be possible. People are able to control themselves and their inner self. This means that being in control or not depends on the self. Controlling the self indicates that one cannot be affected by whatever happens on the outside. Self-discipline is the solution to self-control. In this case, an individual decides how the circumstances affect him. Hence, it is possible to be happy regardless of what is happening, good or bad. Indeed, it is possible to control the circumstances rather than allow the circumstances to control you.
The soldier is encouraged to live a heroic life. He should also be ethical, seeking to promote pleasure and minimize pain. This is possible from a Stoic perspective. Such a soldier is relevant in the 21st century as he was during the time of Epictetus. Besides having an ethical life, it teaches how to live according to the dictates of nature. The teaching is critical for personal development. It allows for one to develop the ways of achieving self-development by learning self-control regardless of the situation one is facing. It helps me personally to become more mature such that I do not allow the circumstances to control my happiness.