"You cannot lead by memo, you cannot lead by shouting, you cannot lead by delegation of your responsibility - you must lead by example."
-General Charles Krulak
Any discussion of leadership should first start with how it is defined currently and historically. Marine Corps Doctrine and other service publications offer three definitions of leadership. Two, as defined in the doctrine MCDP 6 Command and Control, are couched in what can fairly be described as broad, academic terminology .
The third definition, found in the glossary of the Marine Corps Manual (1980) under the title “Leadership, military” comes directly from General John A. Lejeune, and is the definition with which Marines are probably best acquainted. General Lejeune stated that leadership is “the sum of those qualities of intellect, human understanding and moral character that enable a person to inspire and to control a group of people successfully.”
Marines find this definition most helpful historically and to this day because it speaks not only to the objective, i.e., to “control a group of people successfully,” but also to the means that give rise to good leadership in action, i.e., the sum total of “intellect, human understanding, and moral character” that enables successful inspiration, command, and control.
It is implied that Lejeune's definition is an admonition for personal and professional development – all Marines should seek to develop their intellect, human understanding and moral character to the greatest degree possible – in order to become better leaders of Marines.
Note:  MCDP 6 Command and Control defines leadership as “the influencing of people to work toward the accomplishment of a common objective."