The Black Hills, a cluster of primordial rock fingers and forested knolls in western South Dakota and eastern Wyoming, is so called for the dark shade of its ridges. From space, it appears as an evergreen birthmark in the center of North America, a patch of dense texture on the great, golden plains. It is the oldest mountain range in North America, a sacred site for many Indigenous Americans and, as illustrated in the new documentary Lakota Nation vs the United States, the “cradle of civilization” for the Lakota people.

The Lakota fought to protect the Black Hills, and won; in April 1868, members of the US government’s Indian Peace Commission agreed to treaty terms at Fort Laramie, in present-day Wyoming, which pledged on the constitution that the Great Sioux reservation – all of present-day western South Dakota, as well as parts of North Dakota, Wyoming, Montana and Nebraska – would be “set apart for the absolute and undisturbed use and occupation of the Indians”.

It took eight months to gather various Indigenous leaders to translate the terms of the treaty, and collect or coerce 135 signatures. The documentary, directed by Jesse Short Bull and Laura Tomaselli, sharply contrasts the lush beauty of the Black Hills with the stark, impenetrable language and deadening volume of the legal proclamations used by the US to whittle away Indigenous land, autonomy, freedom. “X marks the verbose legal language used to distract or dull the senses,” says the Oglala Lakota poet Layli Long Soldier in the film’s solemn, hard-bitten narration. “X marks the swindling made possible only because of a language they could not read or write.”

The Fort Laramie treaty is one of hundreds forged by the US government to both burnish its reputation as a negotiating power and to expand through the forced removal of Indigenous people. It’s one of the hundreds summarily violated. The film clicks through page after page of treaty document referencing and overriding and nullifying former treaty documents – a “muddy switchback trail to follow,” says Long Soldier, a cascade of ever-mutable traps.

It is also the legal foundation, upheld in US court, for decades of activism to reclaim the Black Hills. “The mantle of our treaty has always been held, every single generation since the time of warfare,” Short Bull, who is Oglala Lakota, told the Guardian. For the federal government, “it’s been relegated to a dusty old artifact of non-relevance. But to our people, it was something. We always held it in the highest regard, and always had people committed to keeping the United States on its end of the bargain.”

Lakota Nation vs United States takes a sweeping look at this injustice. There’s the violation of the Fort Laramie treaty as part of a pattern of land theft, which began shortly after it was ratified, once gold was reported in the Hills. The film punctures the revisionist history glorifying the “last stand” of Gen George Custer, whose entire regiment was wiped out by a band of Lakota, Cheyenne and Arapaho in the Battle of Little Bighorn, in 1876. Contrary to American textbook narratives of people inevitably cowed by technologically dominant Europeans, the Sioux Nation – a collection of tribes called Očéti Šakówiŋ, encompassing the Lakota, Dakota and Nakota peoples – were in a position of strength relative to the US. The narrative of the Battle of Little Bighorn as some sort of American triumph, or hallowed martyrdom of Custer, “is actually being told by the losers of the battle,” said the actor Mark Ruffalo, an executive producer of the film. “It really was an invading force that was trying to commit genocide. And they were beaten. The reason for these treaties is because of the strength of the Lakota, Dakota, Nakota people, not because of their weakness.”

“This was a way to live in peace between two sovereign nations, and it was as much in control of our Native brothers and sisters as it was the United States government,” he added. “And it was the United States government who consistently, over and over again, broke those treaty agreements.”

The Indigenous victory at Little Bighorn spurred the US to pass a measure, tucked into an appropriations bill, illegally seizing the Black Hills, confining the Sioux to reservations a fraction of the size promised. Americans hunted the buffalo to near extinction, depriving Indigenous people of their main source of food. Their youth were forcibly assimilated via abusive boarding schools. To add insult to injury, the federal government funded a memorial to US presidents, a contested “shrine to democracy” known as Mount Rushmore, carved on to a sacred mountain. The film offers “an alternative history to the one that I’d been taught,” said Ruffalo, who has protested the Dakota Access Pipeline alongside Indigenous activists at the Standing Rock reservation northeast of the Black Hills. “There was a great injustice that’s happened here, probably one of the deepest injustices that America has not only inflicted on its Native brothers and sisters but inflicted on itself. And now this is time for making that right.”

The supreme court has recognized the injustice, if not offered suitable restitution. In 1980, the court upheld the sanctity of the treaty and awarded the Sioux $106m – about $1-$2 an acre – in compensation. The Sioux have refused the reward, which still sits in a US treasury account accumulating interest and is now worth about $2bn. “Those billions of dollars would mean a lot,” said Ruffalo, referring to the many chronic issues – addiction, mental illness, poor health, the crisis of Missing and Murdered Indigenous Women – plaguing reservations, which were constructed, as the film says, to subdue a people. “But the nature of the relationship to this land far exceeds any amount of money or gold the United States might want to heap on these people.”

The Black Hills claim dovetails with the modern Land Back movement across the US and beyond to restore land to Indigenous people, covered in the film’s final section, which works to defuse white American fear over a retributive land grab. “I think a lot of white Americans sort of in their heads fear the behavior that their ancestors or colonists did toward Indigenous people, that will happen back to them, like that karma will come back around,” said Tomaselli, the film’s co-director. But “at the end of the day, it’s not about property,” said Short Bull. “It’s not about a barbed wire fence, something that’s ‘this is mine, this is yours.’ It’s fundamentally looking at land as something that you are a part of and hold a responsibility to look after.”

There is precedent for a land return – several members of the Great Sioux Nation were able to purchase Pe’ Sla, a sacred Black Hills site in 2012, which is now in federal Indian trust status. In 2018, the Northern Cheyenne tribe of Montana and the Arapaho tribe of Oklahoma purchased 1,020 acres (or about 1.59 square miles) of land near Bear Butte, a bluff and state park northeast of the Black Hills, for $2.3m. “We’re not saying to people, ‘Yeah, you did this!’ That’s not the approach,” said Ruffalo. “The approach is, ‘You know what, there are decisions being made every single day that affect these issues, and there were decisions that were made that made these issues happen. We’re not the people who did it, but we can be the people who right it.”

For such a long trail of injustices, the film’s prevailing tone remains resilient and assertive, if chastened. “I’ve seen a lot of tragedy, a lot of heartache, a lot of pain, and I’ve seen a lot of my relatives carry a lot of things on their shoulders,” said Short Bull. “Our people are used to getting dirt kicked in our face by the federal government. We’re used to being told no. We’re used to being negated to something other than human. That’s not a surprise. We know that very well. But we still hold that optimism, and we know that sometimes it takes awhile.”

• Lakota Nation vs the United States is in US cinemas on 14 July with a UK date to be announced