The Battle of Gettysburg is one of the most memorable—and significant—moments in American history. As evidenced by countless books, classes, documentaries, reenactments, memorials and movies, the Battle—and President Abraham Lincoln’s now famous Address four months later—live on in American collective consciousness. Today, 150 years later, the historical weight of the Battle is as clear as it was shortly after the first shots were fired on July 1, 1863. As soldiers on both sides knew, history was being made during the three-day battle.

As the president realized, the speech at Gettysburg was an opportunity to speak directly to his fellow citizens about the importance of the war: to explain why the conflict was worth fighting and must continue to be fought. Earlier that year, in a letter –  intended to be read publicly – to Illinois friend James Conkling, Lincoln  wrote:

“There are those who are dissatisfied with me. To such I would say: You desire peace; and you blame me that we do not have it. But how can we attain it?”

The Union that the president now vowed to save was not the same Union the nation went to battle for nearly three years before; the emancipation of more than three million black slaves (and the enlistment of thousands of black soldiers) had redefined the purpose and tenor of the war. He would not— he could not— consider the possibility of a negotiated peace with the Confederates. His address at Gettysburg would reinforce these facts and the reasoning behind them while framing the stakes of liberty, democracy and equality that Lincoln now understood constituted the essence of the struggle.

Lincoln would not fully understand how his words ultimately inspired an awakening throughout the nation, stirring the hearts and souls of millions of Americans throughout the 150 years since and compelling us forever more to think of Gettysburg as hallowed ground. The democratic ideals of the American system of government Lincoln championed at Gettysburg provided comfort and inspiration during the darkest days of the Great Depression and the world war that followed, and were a rallying cry for the civil rights movement of the 1950s and 1960s.

In fact, it had become so embedded in the American psyche that when Martin Luther King Jr. opened his “I Have a Dream” speech with an allusion to the Gettysburg Address, the 250,000 people gathered for the March on Washington in August 1963 recognized it instantly. Speaking from the steps of the Lincoln Memorial, upon which the words of the Gettysburg Address are inscribed, King took the nation to task for the promises of equality – made “five score years ago” – that remained unfulfilled.

When the first shots of the Civil War rang out, no one could have imagined the tragedy and terror it was to bring, or what it would ultimately deliver – an end to slavery and a new definition for American democracy. Today, we continue to pay reverence to Lincoln – the individual who guided the nation through its greatest crisis; the man who redefined our country’s future through the force of his leadership and intellect. At his Memorial in Washington, D.C., behind Daniel Chester French’s imposing figure of the 16th president, the words of the Gettysburg Address are inscribed as a reminder of his influence on the country. Prior to the Civil War, the United States was a cooperative of states. The war, as Lincoln pronounced at Gettysburg, gave us a “new birth of freedom” and the United States was reborn as a singular national body. Over the last century and a half, Lincoln’s words have become, and remain, a symbol of America’s progress and promise.

Nancy Koehn is an historian at the Harvard Business School, where she holds the James E. Robison Chair of Business Administration.

Required Listening: Nancy Koehn breaks down what the Gettysburg Address means  150 years later.