It was just after the Revolutionary War — at a time when a young United States and a mighty Spain were fighting for control of North America — that American war hero James Wilkinson marched across the border between the U.S. and Spanish-ruled Louisiana to sign a secret allegiance to Spain.

"Some people are born to treachery," author Andro Linklater tells NPR's Steve Inskeep, "and I think he was."

The most notorious American traitor you've probably never heard of, Gen. James Wilkinson is the subject of Linklater's book An Artist in Treason: The Extraordinary Double Life of General James Wilkinson.

Wilkinson's betrayals began after his service in the Continental Army ended. He crossed the Appalachian Mountains and joined the masses of pioneers eager to profit in the fertile Kentucky fields. He became a land speculator — and then, like many of his fellow homesteaders, fell promptly into debt.

But Wilkinson had charm — it would come in handy later, in his career as a spy — to help him survive.

"There was a guy, very old-timer, and he said he could remember Wilkinson being called on by somebody he owed money to," Linklater says. "The conversation went on all afternoon, and when it was over, the guy Wilkinson owed money to came away having lent him another $500."

Soon enough, though, Wilkinson had found another, more lucrative form of survival: selling American secrets to the Spanish empire.

"As a spy, his tradecraft was excellent. Wilkinson sent his information in a code — just rows of numbers in groups of four," Linklater says. "It was never broken."

Through these coded messages, Wilkinson informed the Spanish of the Lewis and Clark expedition and its secret goal of finding a land route through the Western mountains and to the Pacific Ocean. He suggested that his paymasters send armed patrols to intercept the expedition — which the Spanish did.

"Only by the grace of God did they fail to find them," Linklater says. If the Spanish patrols had been luckier or more skillful, we might remember Lewis and Clark — if we remembered them at all — as two explorers who vanished in the West. And the course of American history might well have been dramatically altered.

Wilkinson also recommended that the Spanish build huge defensive lines to prevent American expansion westward after the Louisiana Purchase — again, advice that Spain acted upon.

Silver Dollars Everywhere, Including In The Drink

The real problem for Wilkinson was getting paid for his intelligence, which was the only reason he had become a spy in the first place. Wilkinson was hard pressed to find a way of secretly transporting the loud, unwieldy payments the Spanish sent him — thousands of silver dollars at a time. He tried packing them in casks used for sugar, coffee and rum, but the clinking of the cash made it hard to hide the barrels' valuable contents.

"On one occasion, one of his messengers, who was carrying about 3,000 silver dollars, was murdered by his boatmen," Linklater says. "At that moment, Wilkinson absolutely came to the very edge of being discovered."

The five murderers, all Spanish, took the money and scattered across the Kentucky countryside, but were soon captured and taken before a magistrate. As luck would have it, though, the assassins spoke no English — and Thomas Power, the interpreter the magistrate sent for, was secretly another Spanish spy.

With Power there to translate, the Spaniards explained that the money they had stolen was in fact a payment for information Wilkinson had sold to Spain. But what Power told the magistrate, Linklater recounts, was: "'They just say they're wicked murderers motivated by greed.'"

And just like that, Wilkinson got away.

In Whom Do We Trust?

In fact, Wilkinson got away with a lot, considering how many people distrusted him.

"Almost everyone suspected him of passing information to Spain," Linklater says. "Every single president from Washington to Madison knew of people's suspicions about him. And yet they all trusted him."

And while his indecipherable code made it impossible to link him to the leaks, it couldn't stop the accusations.

"His career virtually ended [eventually], because one after another the rats come out of the woodwork," Linklater says. "They all betray him — left, right and center — but they can't find his code. They can't break the code, they can't prove that he got paid. He faced about four or five public inquiries, two or three court martials, and each time he was found not guilty."

It wasn't until the 20th century, long after Wilkinson's death, that historians finally uncovered documentation of his connection to the Spanish empire. So as far as Wilkinson himself was concerned, he pretty much got away with treason.

Linklater says the story of a man who walked among the Founding Fathers while simultaneously plotting against them is invaluable because of how it reminds us that the United States was not always destined for greatness.

"To see history through a villain's eyes rather than through the heroic eyes of the Founding Fathers," he says, "is to see just what dangers did face the United States — and what a fate it did escape."

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