It has launched illegal and unjust wars with catastrophic human consequences; it has helped overthrow democratically elected governments; it arms and backs some of the most brutal dictatorships on the face of the earth; and it has a track record of supporting terrorist organisations. Even many of its ardent supporters admit that the US foreign policy elite has a somewhat chequered history.

Today, an American hero stands in the dock, damned for a relatively tiny ray of light he shone on the darker recesses of this elite. Over three years ago, US soldier Bradley Manning – even now just 25 years old – leaked 250,000 US diplomatic cables and half a million army reports. There has never been a bigger leak of classified material in the history of the United States.

His punishment has already been severe. According to Juan Méndez, the UN special rapporteur on torture, he has faced cruel, inhuman and degrading treatment. For months, he was deprived of human contact. He was stripped of his clothes, left without privacy, and forced to sleep without any darkness. In 2011, P J Crowley was forced to resign as the US state department’s official spokesman after slamming Manning’s treatment as “ridiculous and counterproductive and stupid”.

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Manning now begins a military trial, charged with a capital offence, though the prosecution promise not to seek the death penalty, leaving him facing 20 years in prison. As two US champions of the First Amendment on free speech, Floyd Abrams and Yochai Benkler, have written: “If successful, the prosecution will establish a chilling precedent: national security leaks may subject the leakers to a capital prosecution or at least life imprisonment.”

Manning is partly being tried under the Espionage Act, a piece of legislation dating back to the First World War. He faces 22 charges in total: to 10 of them he has pleaded guilty, including wilfully communicating to an unauthorised person. But the most alarming charge is that he was “aiding the enemy” – in other words, that he intentionally helped al-Qa’ida.

No wonder powerful interests in the US want to make an example of Manning. Among the videos he released was an Apache helicopter conducting a bombing raid that killed Iraqi civilians and a Reuters journalist. “The most alarming aspect of the video to me was the seemingly delightful bloodlust they appeared to have,” Manning has said, appalled by the lack of “value for human life” shown by the pilots’ descriptions of “dead bastards”. Here was the “on-the-ground reality” of both the Iraq and Afghan wars, he claimed.

The truth is Manning has done a great service, both to the American people and to the world as a whole. US foreign policy depends on secrecy, not simply because of fear of US enemies, but because the reality would often horrify the American people.

Back in the 1970s, my parents were among South Yorkshire families who took in Chilean refugees fleeing General Pinochet’s dictatorship. One was a woman with two kids; she had been raped, her husband murdered. She ended her life by flinging herself off a Sheffield tower block. Pinochet’s bloody junta had come to power on the back of secret CIA aid: as Henry Kissinger said before the democratically elected socialist President Salvador Allende was overthrown: “I don’t see why we need to stand by and watch a country go Communist due to the irresponsibility of its people.” Under Reagan, US-backed right-wing terrorists went on a frenzied rampage across Central America: there was an intentional conspiracy to keep this horrendous reality away from the American people.

This was a long time ago, some will say; it was the Cold War, after all, and normal rules were suspended. That’s probably little comfort to those still grieving the Disappeared, and indeed there is substantial evidence of US involvement in the more recent 2002 coup against Venezuela’s Hugo Chávez. But there has been a striking continuity in US foreign policy since 1898, when the American Anti-Imperialist League – dominated by an old guard horrified at a slide towards European-style colonialism – opposed the bloody invasion of the Philippines.

There has always been a somewhat Orwellian quality to US foreign policy: “we have always been at war with Islamic fundamentalism”, for example. And yet in the 1980s, US arms were distributed through Pakistan’s secret services to the Afghan mujihadeen: they were freedom-fighters, you see. Then we ended up in a never-ending war in Afghanistan, battling on behalf of a corrupt and undemocratic government, against Islamic fundamentalist elements. Several hundred miles away, the US is proactively backing Syria’s jihadists alongside its Islamist fundamentalist ally, Saudi Arabia. Waves of Islamist fighters were recruited by the calamity of Iraq.

There is nothing patriotic about the poorly scrutinised actions of the US foreign policy elite. Scores of young men or women are sent to be killed or maimed: those who call for bringing them to safety are smeared as “unpatriotic”. US civilians are put at risk of “blowback”, a CIA word for the unintended consequences of foreign interventions. They can even fail disastrously on their own terms. Back in the 1950s, the US helped overthrow Iran’s last democratically-elected Prime Minister, Mohammad Mosaddegh, fuelling anti-American sentiment that helped drive the Iranian Revolution.

That is why Manning has done us such a service. He has encouraged us to scrutinise the hidden realities of US power, and consider the dire consequences of decisions shrouded in secrecy. His actions should compel us to build a more open, balanced world, where great powers are less able to throw their poorly understood weight around. It would be a long-term investment: the US is in long-term decline, and autocratic China may take its place, quite possibly using its power more unjustly. Better, then, to challenge this world order now.

I happen to believe the creation of such a world is not a naïve fantasy. It can and must be built. And however your trial goes, you, Mr Manning, will be remembered for your own contribution in building it.