This rocky, somewhat forbidding hill in Pennsylvania is called Little Round Top. Few people realize it, but the United States of America, and possibly the whole of democratic government and freedom as we know it today, was saved here. I think it’s one of the most important sites in American history, right up there with Independence Hall, the birthplace of Lincoln or the White House.

150 years ago today, on July 2, 1863, Union Col. Joshua Lawrence Chamberlain and his beleaguered 20th Maine troops held the top of this hill, repulsing wave after wave of troops who were attacking from the bottom of the hill. Despite his men being exhausted, outnumbered and low on ammunition, Chamberlain not only held his position, but he made a very strategic decision: as the final wave of Confederates approached, he ordered his men to advance and swing down the hill, sweeping the enemy downward and preventing them from getting around the side of his forces. This was a textbook maneuver.

Chamberlain’s order literally saved the Union. If his forces had been flanked and Little Round Top taken, the Confederates under General Lee probably would have won the battle. This could have changed the course of the war and of history.

It’s difficult to speculate what would have happened if the South had won the Civil War. A lot of writers have imagined that scenario. I do believe that, had the U.S. been splintered in the 1860s and the institution of slavery continued in the South, all Americans–North and South–would be much less free today than they are.

I’ve been to Gettysburg and I climbed Little Round Top twice, once in 1988 and again in 1995. It’s a pretty steep and harsh hike, especially in the summer. I couldn’t imagine doing it in a wool Civil War uniform carrying a musket and 60 pounds of equipment on my back. With people shooting at me.

Read more about the second day of Gettysburg here.