Some of the greatest leaders in history were Stoics -- Cicero, Epictetus, Seneca, Marcus Aurelius. It's no surprise considering Stoicism's emphasis on self-mastery. Success and leadership has to start with looking in the mirror.
The key is found in responding -- not reacting to external circumstances. Stoics taught although we can't control what happens to us in life, we can control our perception of it, which makes all the difference.