I think that it sounds like you are seeking a more contemporary turning point. In this, I would suggest that the terrorist attacks on September 11, 2001 would be a significant turning point in the history of the United States. I find it difficult to see any other argument but the idea that there was a world before that fateful morning and one after it. I would see it as a turning point because the America that was constructed after the attacks of 9.11 is a fundamentally different one than the one before it. The adoption of the Patriot Act, the development of the news crawl on the bottom of the news telecasts, the "War on Terror," and the notion of being "more vigilant" are all elements of life that have been the children of that day. It can be seen as a turning point in American History because it marked a specific moment where a pivot towards a new consciousness can be seen. The attacks of September 11 brought about a new understanding of the world and America's place within it. It also brought with it the idea that how Americans see themselves in a globalized world was going to be forever changed as a result of it. I think that these would serve as representations as to how a turning point in American History would be the attacks of September 11, 2001.