A "once-in-a-lifetime" archival discovery has revealed how controversial American explorer William Clark plotted a massive land grab from Indigenous nations and broke a peace treaty with Great Britain back in 1816.

The map was hiding under a false identity and was only attributed, re-dated, and decoded thanks to the work of University of Cambridge historian Dr Robert Lee.

Lee saw the map in microfilm under the authorship of one Captain Eli B. Clemson, but the scholar saw that the map didn’t make sense when considering the Osage treaty of 1808 – where the Osage Nation ceded all the land east of the fort in Missouri and Arkansas north of the Arkansas River to the United States.

Clark, one of half of the famous Lewis and Clark expedition that explored the interior of the US from 1803 to 1806, was once revered as an American "hero", but has come under increasing controversy as his reputation as a "friend to Indians" has been discredited. 

As reported in the journal William and Mary Quarterly, the map shows a massive land grab. Clark, the then governor of the Missouri territory, used the map to add 10.5 million acres of Sauk, Meskwaki, and Iowa territory onto the United States – that’s an area equivalent to Switzerland.

“This astonishing map shows how William Clark leveraged the US–Indian treaty system to promote settler supremacy in the United States at a time when he’s been praised for trying to protect Indigenous land from squatters," Lee said in a statement. "Now we can see just how scheming and disingenuous he really was.” 

The researchers explained how the style, spelling, and symbols are consistent with Clark’s work – but it is a particular line that is key to the re-established authorship. The map, lost in 1951, was accompanied by a letter by Clark describing the line between the Arkansas and Red Rivers.

This line is a unilateral redrawing of the 1808 treaty, taking half of today’s Missouri from the Indigenous people living in the region. It also broke the Treaty of Ghent, which put an end to the War of 1812 with Great Britain. The official order was to restore the pre-war boundaries, including those with the Indigenous Nations.

“A naïve interpretation might say he found a huge loophole in the Treaty of Ghent. A realistic one would say he broke it to seize a landmass triple the size of Connecticut,” Lee explained.

“Clark’s land grab worked by denying that his post-war interpretation of the Osage treaty was new. He carefully maintained the fiction that he had clarified an old boundary, not manufactured one. This plan worked so well that historians have tended to either believe him or overlooked the incident entirely.”

The new (and cheap) land attracted thousands of emigrants, many of them slaveholders. The increase in settler population led to Missouri becoming a state in 1821 and the claims of Sauks, Meskwakis, and Iowas were ignored. Clark paid the Nations half a cent per acre in 1824, while the territory was selling for 4 to 12 dollars an acre.

Clark is believed to have been responsible for the taking of 419 million acres of Indigenous land. That’s roughly one-third of all the land claimed by the US by the time of Clark’s death in 1838.

The land grab contributed to generations of hardship for the Sauks, Meskwakis, and Iowas.