THANKSGIVING AS we know it was first celebrated 153 years ago, when the nation was divided by a savage civil war. Abraham Lincoln proclaimed the new national holiday to help foster a sense of unity in a country torn between its regions and within them. But he also had a greater idea in mind, which had been stated just a week before Thanksgiving, in his speech at Gettysburg honoring the thousands who had died there some months before. It was the idea that, with the Emancipation Proclamation, a new, truly democratic nation could be born and, more important, that its ultimate victory would stand as a beacon to the embattled forces of freedom and self-government throughout the world.

As the historian Sean Wilentz notes, “in the middle of the nineteenth century, democracy was in retreat.” France had gone through a series of upheavals and reverted to monarchy. Numerous uprisings for self-rule and democracy had been put down all across Europe, after which many thousands fled to these shores. (A half-million immigrants served in the Union Army.) The truth, as Lincoln saw it, was that a democracy could not survive and prosper on its own — that the ideals it stood for needed to have widespread assent among the nations and that the United States must stand as the foremost exemplar of these principles.

In the century-and-a-half since the first national Thanksgiving, America has played this part with varying degrees of enthusiasm. But the lesson finally learned from two world wars and one world-shaking depression was compelling and decisive: that narrow nationalism, racial and religious bigotry, xenophobia and barriers to international commerce are capable of producing catastrophe on a global scale. After the greatest bloodletting in human history, this country committed itself to active engagement with the rest of the world and support of like-minded societies, a generally bipartisan attitude that has endured to this day.

Now there is a whiff of 1930s Europe in the air — the rise of authoritarian governments and “ethno-nationalism,” to use the putrid term favored even by some in this country. A retreat from our long-standing role as an example and advocate for free, representative government seems possible.

Ronald Reagan, like other politicians over the years, was fond of repeating the words of the Puritan sermon about a “city on a hill.” That image in turn has its biblical origins in the Gospel of Matthew, where Jesus tells his followers, “You are the light of the world,” and then adds this challenging admonition: “A city set on a hill cannot be hidden.”

As is said in churches throughout the land on this day of thanks, “Amen.”