Presidents & American foreign policy

Are trends in global politics simply the result of large impersonal structural forces, or does human agency matter? For example, would America have become the dominant country in the 20th century regardless of who was president? To what extent did leaders matter, and if so, what kind of leader? Leadership theorists focus their attention on what they call transformational leaders, but were "transformational" leaders like Woodrow Wilson and Ronald Reagan really more important than "transactional" leaders like Eisenhower and the first Bush? What are the lessons we can draw for leadership and foreign policy in this century?"