In the relatively short history of the United States, there have been many turning points and landmark movements that irrevocably altered the direction of the nation and signaled the dramatic start of a new historical reality. Some took the form of groundbreaking political and philosophical concepts; some were dramatic military victories and defeats. Still others were nationwide social and religious movements, or technological and scientific innovations.What all of these turning points had in common, is that they forever changed the character of America. Sometimes the changes brought about by these events were obvious; sometimes they were more subtle. Sometimes the effects of these turning points were immediate; other times, their aftershocks reverberated for decades. Regardless, these great historical turning points demand to be understood.Edward O'Donnell / Holy Cross CollegeEdward O'Donnell is a professor of History at Holy Cross College. He is the author of several books, including "Henry George and the Crisis of Inequality: Progress and Poverty in the Gilded Age." He frequently contributes op-eds to publications like Newsweek and the Huffington Post. He has been featured on PBS, the History Channel, the Discovery Channel, and C-SPAN. O'Donnell also has curated several major museum exhibits on American history and appeared in several historical documentaries. He currently hosts a history podcast, In The Past Lane.