Prof. Allen Guelzo wrote a July 1 opinion piece in USA Today, "Gettysburg's message to tyrants".
From USA Today:
There are a few moments in history when the normal, unhurried pace of events undergoes a sudden, violent compression, when single decisions become the starts of avalanches and people find the compass points of their lives spinning wildly askew. We have experienced a few of these moments in living memory —Pearl Harbor, the Kennedy assassination — but the largest one in U.S. history was the Battle of Gettysburg, whose150th anniversary looms before us this week.
Gettysburg was the midpoint of our four-year Civil War. But it also became the war's hinge-point, and it's not difficult to see why.