The film Dirty Wars, which premiered at Sundance, can be viewed, as Amy Goodman sees it, as an important narrative of excesses in the global "war on terror". It is also a record of something scary for those of us at home – and uncovers the biggest story, I would say, in our nation's contemporary history.

Though they wisely refrain from drawing inferences, Scahill and Rowley have uncovered the facts of a new unaccountable power in America and the world that has the potential to shape domestic and international events in an unprecedented way. The film tracks the Joint Special Operations Command (JSoc), a network of highly-trained, completely unaccountable US assassins, armed with ever-expanding "kill lists". It was JSoc that ran the operation behind the Navy Seal team six that killed bin Laden.

Scahill and Rowley track this new model of US warfare that strikes at civilians and insurgents alike – in 70 countries. They interview former JSoc assassins, who are shell-shocked at how the "kill lists" they are given keep expanding, even as they eliminate more and more people.

Our conventional forces are subject to international laws of war: they are accountable for crimes in courts martial; and they run according to a clear chain of command. As much as the US military may fall short of these standards at times, it is a model of lawfulness compared with JSoc, which has far greater scope to undertake the commission of extra-legal operations – and unimaginable crimes.

JSoc morphs the secretive, unaccountable mercenary model of private military contracting, which Scahill identified in Blackwater: The Rise of the World's Most Powerful Mercenary Army, into a hybrid with the firepower and intelligence backup of our full state resources. The Hill reports that JSoc is now seeking more "flexibility" to expand its operations globally.

JSoc operates outside the traditional chain of command; it reports directly to the president of the United States. In the words of Wired magazine:

"JSoc operates with practically no accountability."

Scahill calls JSoc the president's "paramilitary". Its budget, which may be in the billions, is secret.

What does it means for the president to have an unaccountable paramilitary force, which can assassinate anyone anywhere in the world? JSoc has already been sent to kill at least one US citizen – one who had been indicted for no crime, but was condemned for propagandizing for al-Qaida. Anwar al-Awlaki, on JSoc's "kill list" since 2010, was killed by CIA-controlled drone attack in September 2011; his teenage son, Abdulrahman al-Awlaki – also a US citizen – was killed by a US drone two weeks later.

This arrangement – where death squads roam under the sole control of the executive – is one definition of dictatorship. It now has the potential to threaten critics of the US anywhere in the world.

The film reveals some of these dangers: Scahill, writing in the Nation, reported that President Obama called Yemen's President Saleh in 2011 to express "concern" about jailed reporter Abdulelah Haider Shaye. US spokespeople have confirmed the US interest in keeping him in prison.

Shaye, a Yemeni journalist based in Sana'a, had a reputation for independent journalism through his neutral interviewing of al-Qaida operatives, and of critics of US policy such as Anwar al-Awlaki. Journalist colleagues in Yemen dismiss the notion of any terrorist affiliation: Shaye had worked for the Washington Post, ABC news, al-Jazeera, and other major media outlets.

Shaye went to al-Majala in Yemen, where a missile strike had killed a group that the US had called "al-Qaida". "What he discovered," reports Scahill, "were the remnants of Tomahawk cruise missiles and cluster bombs … some of them bearing the label 'Made in the USA', and distributed the photos to international media outlets."

Fourteen women and 21 children were killed. "Whether anyone actually active in al-Qaida was killed remains hotly contested." Shortly afterwards, Shaye was kidnapped and beaten by Yemeni security forces. In a trial that was criticized internationally by reporters' groups and human rights organizations, he was accused of terrorism. Shaye is currently serving a five-year sentence.

Scahill and Rowley got to the bars of Shaye's cell to interview him, before the camera goes dark (in almost every scene, they put their lives at risk). This might also bring to mind the fates of Sami al-Haj of al-Jazeera, also kidnapped, and sent to Guantánamo, and of Julian Assange, trapped in asylum in Ecuador's London embassy.

President Obama thus helped put a respected reporter in prison for reporting critically on JSoc's activities. The most disturbing issue of all, however, is the documentation of the "secret laws" now facilitating these abuses of American power: Scahill succeeds in getting Senator Ron Wyden, who sits on the Senate intelligence committee, to confirm the fact that there are secret legal opinions governing the use of drones in targeted assassinations that, he says, Americans would be "very surprised" to know about. This is not the first time Wyden has issued this warning.

In 2011, Wyden sought an amendment to the USA Patriot Act titled requiring the US government "to end practice of secretly interpreting law". Wyden warns that there is now a system of law beneath or behind the law that we can see and debate:

"It is impossible for Congress to hold an informed public debate on the Patriot Act when there is a significant gap between what most Americans believe the law says and what the government is using the law to do. In fact, I believe many members of Congress who have voted on this issue would be stunned to know how the Patriot Act is being interpreted and applied. "Even secret operations need to be conducted within the bounds of established, publicly understood law. Any time there is a gap between what the public thinks the law says and what the government secretly thinks the law says, I believe you have a serious problem."

I have often wondered, since I first wrote about America's slide toward fascism, what was driving it. I saw the symptoms but not the cause. Scahill's and Rowley's brave, transformational film reveals the prime movers at work. The US executive now has a network of secret laws, secret budgets, secret kill lists, and a well-funded, globally deployed army of secret teams of assassins. That is precisely the driving force working behind what we can see. Is fascism really too strong a word to describe it?

• This article originally referred to Scahill and Rowley's documentary as Secret Wars; this was amended to Dirty Wars at 5.20pm ET on 3 February. The phrase "US kill list" in the subhead was also amended to "kill list" in order to remove possible ambiguity