If it had worked out differently, the group of settlers that came to be known as the Donner Party would have slipped over the Sierra Nevada into California—and obscurity.

But poor planning, a series of bad decisions, and early snowstorms caused 60 of the original pioneers to become stranded in the mountains during the winter of 1846. And as hypothermia set in and food ran out, many resorted to that greatest of human taboos: cannibalism.

By delving into the biographies of leading party members and placing them into the religious and political context of the time, he has moved the story of the Donner Party beyond mere sensationalism to reveal uncomfortable truths about America, then and now.

National Geographic caught up with Wallis, a historian of the American West, by phone in St. Louis, Missouri.

Let’s cut to the chase. Did members of the Donner Party eat each other to survive? And what evidence do we have?

There are different types of cannibalism; ritualistic, sacrificial, and survival cannibalism. There’s been survival cannibalism forever, and there was a lot during this period of time. There was maritime cannibalism, polar exploration cannibalism, and it continued on until the 20th century, in the Warsaw Ghetto and Stalingrad. (Read: "Cannibalism Study Finds People Are Not That Nutritious.")

In order to survive, members of what ended up being called the Donner Party did indeed turn to survival cannibalism. Some of the most substantial proof comes from the survivors themselves.

In correspondence, journals, and later, interviews, they freely admitted that when everything else was gone, they turned to cannibalism. They were suffering hypothermia and starvation; they were delirious. But, they knew that out in the snow banks was this great store of protein: people who had already died. They had carefully placed them in the snow banks and that’s what it came down to.

Their own deep freeze?

[Laughs.] Yup, they went to the deep freeze. Over the years, people would ask me, 'What are you working on now?' The short answer was 'A book about the Donner Party.' The long answer was, 'A book about the folly and arrogance of Manifest Destiny, as told through the eyes of its foot soldiers.' But if I’d just say the Donner Party, they’d invariably say, 'Aren’t those the pioneers who got trapped in the mountains and ate each other?' I’d say, 'Yes.' But then I had to explain that that’s only a slice of the Donner pie, albeit an important one.

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In the book, you say, “The Gothic tale of cannibalism draws a real parallel between the individuals consuming flesh and the desire of a country to consume the continent.” That’s a pretty harsh judgment, isn't it?

There are so many metaphors and elements of history that crossroad at this particular point from 1845 to 1847. The term Manifest Destiny was first coined in 1845, by John L. O’Sullivan of the New York Post, in an editorial. Many people, including politicians of course, and others interested in the commercial interests of the U.S., came to the conclusion that God Almighty had mandated the Anglo-Americans as the Chosen People and it was their destiny, their manifest destiny, to take over the entire continent. (Read how a 2,000-mile trail helped define the American mindset.)

Timing couldn’t have been better. We had a bellicose, expansionist president, James Polk, who schemed up a convenient war with Mexico, which owned much of the land we were to take in the West. The story line was, 'There are no people out there, anyway, so let’s take this land!' Of course, there were a lot of people out there, like the Mexicans, and tens, if not hundreds of thousands, of Indians. What we did was gobble up nations.

The most infamous member of the party was a German emigrant named Lewis Keseberg. Give us a bit of background—and describe his heinous deeds.

For me, there are no shining heroes or demons in this story. Keseberg was made into the master villain of this whole tragedy, and he didn’t help his own cause. He and his wife, Philippine, came from Germany. He was a son of a Lutheran clergyman, and they decided to join this vanguard moving west. He was a sharp-tempered fellow, who was sometimes abusive to his young, pregnant wife. He was also accused of plundering Indian burial sites.

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When the fourth rescue party reached him in April 1847, he was the only survivor. He was reportedly found with a cauldron of cooked flesh and discarded bones. There were even rumors from some of the surviving children that he had taken one lad to bed with him to comfort him and the next morning the boy was dead, hung up on the wall of the cabin, like a slab of meat, and later eaten.

The journalists of the day feasted on all this. Sensationalized stories, often filled with outright lies, [nicknamed] Keseberg "The Human Cannibal." It was said he actually relished the taste of human flesh, and that when rescuers offered him alternative protein, he refused it, saying, 'Oh no, I like this better.' Many of those stories are suspect. So, though I don’t think Keseberg is someone to champion, I do believe he got a fairly raw deal.

You say there were no heroes and heroines in this grisly tale. But one person who stands out is Tamsen Donner. Tell us about her—and how women became, as you put it, “she-wolves” to save their kin.

Throughout the many years that separate us from the Donner Party and its tragedy, people have singled out Tamsen as the true heroine. There were many things that she did that are admirable. [A natural educator, she] dreamed of starting a girls’ school in California. She liked to botanize, collecting specimens of plants along the route. For her, the whole trip was a learning adventure.

The reason people flocked to Tamsen is because she totally refused, to her detriment and eventual death, to abandon her husband, George, the elder of the Donner brothers. He had injured his hand near the time they were marooned in the snow. It became gangrenous, and that ultimately claimed his life. She sent her children off to various rescue parties, but she stayed with George until the very end.

One of the surprises in the book is that, if history had turned out differently, Abraham Lincoln might have died with the Donner Party. How did he connect to this story?

That’s one of my favorite stories. For me, James Reed is the most interesting character. He was this entrepreneurial Irish immigrant who built a business in Sangamon County, Illinois. In Springfield, the state capital, there was this astute young prairie lawyer who helped Reed in various business matters. They had been messmates in the Black Hawk War, were good friends, and when Reed declared bankruptcy and set out on the trail to rebuild his life, it seems this young lawyer, Mr. Lincoln, was very interested in going as well.

Lincoln was interested in California for the entirety of his life. He was even offered civic office out in the Pacific Northwest. He might have signed up for the Donner Party, but he had a driven, and often obstinate wife, Mary Todd Lincoln. She already had kinsmen in California, who had gone out in earlier wagon trains. But at the time of the Donner Party, she had a young toddler son and was pregnant with another. Lincoln was also just beginning his political career, after being elected to his one term of Congress. So he didn’t go. But Mary Todd was there waving goodbye at the Donners’ departure point, which is today well marked in the heart of Springfield.

How did the survivors of the Donner Party, particularly children, come to terms with what they had done and seen? Did they go on to live normal lives—or were they too scarred by their experiences?

Eating human flesh was a total, last resort. People say, 'Oh, those cannibals, how could they do that?' I turn it around and say, 'What would you do if you are a mother watching your children starve and freeze to death? You’ve already eaten the horses and oxen, and boiled their hides into a horrible gelatinous concoction; you’ve eaten field mice and finally cut the throats of your beloved family dogs and eaten them, paws and all. But you know that there’s protein that will keep you alive in those snow banks.'

It didn’t really scar the children because they were told to eat it and they knew it kept them alive. Some of them never ever spoke of it again. Some denied it, but not that many. A lot of them went on to live perfectly normal and successful lives, like James Reed, who became a prosperous citizen and business leader in San Jose, California.

The reason I was able to capture a lot of information that’s never really been discussed before is because I struck up a relation with the descendants of the Donner Party. Not many people have talked to the descendants. But they have a great deal to say!

They’re wonderful people, from every walk of life, and I found who were willing to talk and share some of their archival material. There was no guilt or embarrassment. They know they’re part of history, they’re bright people with a great sense of humor, which is very important.

What did you learn about human nature—and America—when writing this book?

It proved several things to me. This grand expansionist movement came at a critical time, when America was fixated on extending its borders. It was, as the great historian, Bernard Devoto, called it, The Year of Decision. Yet I found many of the decisions were not good decisions. I also found solid proof that the old, somewhat clichéd statement about repeating history—that those who don’t learn from history are doomed to repeat it—is true.

The story of the Donner Party, which looms so large in America’s folklore, is not only a metaphor for Manifest Destiny, but also a microcosm of the U.S. Recently, that deadly combination from the past has reared its head again. The two words that come to mind to describe the present are ignorance and arrogance. So my hope is that this story has relevancy for today.

This interview was edited for length and clarity.