Military leadership is a journey, not a destination. It is continually challenged, and must continually prove it self anew against fresh obstacles. Sometimes those obstacles are external events. Other times they are the doubts of those being led. Still other times they are a result of the leaders's own failures and shortcomings.
Political power and influence are different. Once certain levels have been reached, there is no need to prove leadership or competence. A person with such power is accustomed to having every word carefully considere, and every whim treated as an order. And all who recognize that power know to bow to it.
A few have the courage or the foolishness to resist. Some succeed in standing firm against the storm. More often, they find their paths yet again turned form their hopes for goal.