When we try to decide on the most destructive aspects of the legacies of George W Bush and Tony Blair, the Iraq war obviously features prominently. But there are other contenders. Will we most regret the reintroduction of abusive interrogation, previously thought to have been laid to rest by the UN Convention Against Torture? Or will the stampede towards greater governmental secrecy in the end do more damage?

I suspect the winner will be governmental secrecy. Blair retrospectively described his support for the Freedom of Information Act as his greatest mistake in government. That was both extraordinary, and extraordinarily sad: to be sure, complying with the act can be an imposition for public servants, yet even FOIA requests did not reveal the extent of the parliamentary expenses scandal. Most references about duck islands and even criminal offences were redacted out of the FOIA disclosures – we learned about these only through a leak. The fault with FOIA is not its breadth, but its limitations.

Most of the clamour for secrecy involves the conflation of protecting "national security" or "privacy" with preventing "political embarrassment". There is also a vicious circle: we fail to see the perils of secrecy because the secrecy laws make it almost impossible to describe the danger.

I have the privilege of a security clearance in the United States. I take my obligations under the relevant laws extremely seriously and I never violate the rules that govern my access to the prisoners that I represent in Guantánamo Bay. That said, I have been aghast over the last eight years to see the kind of drivel that is considered "classified". One example, discussed in some detail in Ian Cobain's book, and about which I can talk somewhat openly, is the case of Binyam Mohamed. Because the UK security services had indisputably been "mixed up in the wrongdoing" of Binyam's torture, the UK courts ordered that certain documents be revealed to me, as his US lawyer, to help us secure him a fair hearing. Seven paragraphs describing the bare bones of these materials were eliminated from the judges' original opinion based on the vehement objection of the British government.

Ultimately, the judges made these paragraphs public. The judgment made clear that the documents contained far greater detail of Anglo-American misconduct – details that presumably corroborated Binyam's claims of torture. Rather than hold a trial on these issues, the US shipped him back to Britain; rather than risk further embarrassing disclosures, the UK government settled Binyam's civil claim for damages.

The sequel to this episode was depressing: the coalition government has pushed forward with the euphemistic justice and security bill (more accurately called the secrecy bill). This would ensure that no such embarrassing judicial revelations ever occur again. We are told that we have no right, for example, to know how British officials worked with Colonel Gaddafi to help the ogre torture his opponents in Libya. We are told that the British public has no right to know what policy governs when the MI6 shares targeting information for the US "kill list" in Pakistan – that would be a national security issue, albeit one that involves conspiring in an international war crime.

This is where Cobain's readable book makes such a vital contribution to our evaluation of how Bush and Blair – and their heirs – have thwarted the march towards democratic openness. Cobain and I once shared the comfortable notion that Britain stood above the nastiness of torture. Cobain's book demonstrates how naive we were.

One fault of the Bush-Blair years was the politicians' failure to learn basic lessons of history: torture did not secure reliable information in 1600 (when witches "confessed"); it was no more helpful in 2001. Cobain fills in that history in a way that should be required reading for those contemplating a vote for the secrecy bill. He demonstrates a pattern that flows, sadly, throughout the 20th century and into the 21st.

During and just after the second world war, we hated and feared Germans, so we tortured them. Interrogators were told that "mental pressure but not physical torture is officially allowed." While murder was forbidden, interrogators were told they "were permitted to threaten to kill prisoners' wives and children", techniques that were deemed "quite proper". The interrogators read between the official lines, just as their counterparts did later in Guantánamo Bay and Abu Ghraib. They employed stress positions (standing up for eight days on end), strappado (hanging from the wrists, originally devised by the Spanish inquisition) and denial of food, combined with the "standard sleep deprivation and isolation regime". In a precise parallel with Bagram air base, two prisoners died in the custody of one Captain John Smith.

In the British torture prison at Bad Nenndorf, when a prisoner complained that he was going to starve, Captain Smith replied with sang froid: "Yes, it looks like you are." More than 60 years later, Cobain tracked down one German victim, a tough retired businessman, who began trembling with fear when he learned the subject of the proposed interview.

What did this systematic abuse of Germans achieve? These "interrogations … proved, beyond doubt, that Hitler was dead." When the political mandarins were faced with the horror of what had been done to the prisoners, the truth was too embarrassing to bear, so the British authorities made sure there were no public prosecutions where inconvenient truths might seep out. One witness was advised to "escape" (by walking out of the open gate) after being told that if he testified against the British officers he would be the one spending the rest of his life in prison.

Cobain's history continues with British abuses in the late colonial era. As the world sought scientific answers in the 50s and 60s, so the British sought more scientific methods of abusing prisoners in the cold war. Then came the notorious "five techniques" used on the alleged terrorists targeted in Northern Ireland before, most recently, we repeated all the mistakes of history with the "war of terror" (as Borat called it) and Iraq.

We do not, of course, yet know all the sordid details of our recent past. The security services are, with the energetic support of the government, devoted to keeping their misconduct secret for as long as possible. Cobain has used declassified materials from years gone by to demonstrate a pattern that has been recycled with the perceived terrorist threat faced by each generation. His excellent book will, I hope, educate us all.

• Clive Stafford Smith's Injustice: Life & Death in the Courtrooms of America is published by Harvill Secker.

• This article was amended on 6 December 2012, to insert the word "alleged" before "terrorists" in the penultimate paragraph.