In the summer of 1807, the city of Richmond, Virginia, played host to one of the most remarkable trials in early American history. The case involved several legal luminaries, but its undisputed star was the defendant, 51-year-old Aaron Burr.

The New Jersey native had only recently served as Thomas Jefferson’s vice president, but since then his reputation had been marred by political intrigue and his participation in a duel that had left Alexander Hamilton dead.

Burr now stood accused of one of the gravest crimes in American law: treason. According to one account, he had been at the heart of a “deep, dark, and wicked conspiracy” against the young United States.

What was the nature of the plot that had seen Burr charged with treason? Even today, many details of the scheme remain hazy.

“Too many people told too many different stories, and too many people had things to hide,” historian Buckner F. Melton has written.

What is known is that Burr worked to raise a small army on the American frontier. He may have hoped to lead an independent campaign against Spanish-held territories in Texas and Mexico, but it’s also possible that he planned to wrest a portion of the newly acquired frontier from the United States. According to some contemporaries, Burr had designs on founding a new western nation with himself as its emperor.

Burr’s enigmatic conspiracy appears to have originated in 1804—the same year that he shot Alexander Hamilton dead in Weehawken, New Jersey. At the time, Burr’s career was in shambles. Political parties had shunned him, Thomas Jefferson had dropped him as vice president, and the Hamilton duel had left him with potential murder indictments hanging over his head.

Desperate to remake his name, the former Continental Army colonel began plotting a grand military enterprise on the American frontier. After making contact with a British foreign minister named Anthony Merry, Burr floated the idea that Louisiana and other territories west of the Appalachians might be persuaded to secede from the United States.

That August, Merry sent a dispatch to London in which he reported that Burr had offered “to lend his assistance to His Majesty’s Government in any manner in which they may think fit to employ him, particularly in endeavoring to effect a separation of the Western part of the United States from that which lies between the Atlantic and the mountains.”

Britain never took Burr up on his offer—Merry’s letter wouldn’t resurface for decades—but the former vice president continued to plot. In early 1805, he journeyed west and spent several months traveling the Ohio and Mississippi Rivers while scouting territory and recruiting supporters. During one stopover, he met with Harman Blennerhassett, a wealthy Irish immigrant who owned an island in the Ohio River. Upon reaching New Orleans, he made contact with a society of businessmen who favored the annexation of Mexico.

Burr’s allies eventually included dozens of frontier politicians and adventurers, but his most important co-conspirator was General James Wilkinson, the highest-ranking officer in the U.S. Army. Wilkinson had a reputation for duplicity—it would later come to light that he was a paid agent for the Spanish—but he also had vast resources at his disposal. With his frontier troops, he could serve as official cover for any military operations in Mexico or the West.

Burr was careful not to reveal the full extent of his plans to any of his potential recruits, but his movements didn’t go unnoticed. He had attracted attention wherever he traveled on the frontier, and by the time he returned to the East Coast in late 1805, the media was abuzz with rumors. One Philadelphia paper speculated that Burr would soon be “at the head of a revolution party.” It also referenced reports that he planned to “engage in the reduction of Mexico” with the aid of “British ships and forces.”

Despite the controversy beginning to swirl around him, Burr forged ahead with his mysterious plan. In August 1806, he struck out for the frontier a second time and made his way to Blennerhassett’s island, which he intended to use as a rallying point for his forces. Around that same time, he allegedly sent a coded letter to General Wilkinson. “I have at length obtained funds,” it read, “and have actually commenced.”

Wilkinson received the letter that October, but unfortunately for Burr, the general had lost his nerve. Convinced the scheme would fail, Wilkinson betrayed the plot and sent warning to President Thomas Jefferson that a vast conspiracy was brewing in the West. Jefferson was left fuming. He immediately issued a proclamation instructing government officials to quash the frontier plot and arrest its ringleaders.

By December 1806, the noose had begun to tighten around Burr. Militia groups raided his outpost at Blennerhassett’s island while he was away on business, and many of his supporters abandoned the enterprise. Burr had hoped to raise an army of volunteers, but when he finally rendezvoused with his force, it numbered fewer than 100 men.

Undeterred, the former vice president packed the adventurers into flatboats and set out down the Mississippi. He intended to reach New Orleans, but as he neared the city in early 1807, he learned of Wilkinson betrayal and Jefferson’s calls for his arrest. Following a last-ditch attempt to flee, Burr was captured in February near present day Mobile, Alabama. By late March, a posse of guards had brought him to Virginia to face trial.

Almost no one in 1807 knew for sure what Burr had been up to on the frontier, but President Jefferson demanded that he be charged with treason—a crime punishable by death. The case of United States v. Aaron Burr commenced that summer in Richmond. With Supreme Court Chief Justice John Marshall presiding, prosecutors spent several months presenting witnesses against the former vice president. The accounts were often muddled and contradictory, but the prosecution maintained that Burr had been the mastermind behind an attempt to levy war against the United States.

“He was the Alpha and Omega of this treasonable scheme,” lawyer Alexander MacRae proclaimed during one speech, “the very body and soul, the very life of this treason.”

Burr and his team of lawyers—which included two former U.S. attorneys general—mounted a robust defense against the charges. Not only did they prove that Wilkinson had doctored the cipher letter he allegedly received from Burr, they argued that the definition of treason outlined in the Constitution required evidence of an “overt act” from the accused.

When Chief Justice Marshall ruled in favor of this interpretation of the law, the prosecution’s case crumbled. Burr had repeatedly spoken about various illegal schemes, but since he had been absent when his troops gathered on Blennerhassett’s island—the only “overt act” that could be proven—there was no evidence that he had taken up arms against the government. With this in mind, the jury found him not guilty of treason.

Burr walked free in October 1807, but the debate surrounding his actions in the West has continued ever since. Some historians believe he was mounting a filibustering expedition against Mexico and Texas, while others contend that he had more sinister hopes of fomenting a revolution on the frontier. Burr’s own claim at trial was that he was planning to colonize a tract of land in Louisiana. Given his intense secrecy, Burr's true motives may never be known for certain.

“Reaching a final judgment on Burr is difficult,” author David O. Stewart has written. “The confusion has persisted because he had several alternative goals, and because he said so many different things to so many different people.”

Despite his victory in court, Burr was branded a villain in the United States and hanged in effigy in several cities. The disgraced political titan later spent a few years in self-imposed exile in Europe, but returned home in 1812 and established a legal practice in New York, where he lived until his death in 1836. To this day, he remains one of the only major American politicians to have been tried for treason.