President boldest mark on American History

This course has covered American history from the presidency of Andrew Johnson to George W. Bush. In this period, 26 men served as president. In an essay of 500-600 words, which president do you think left the boldest mark on American History? Why? Your essay should present the events that serve as a hallmark to his career and a clear explanation of not only the events but also their impact upon the course of American history.