It is the first serious reversal in almost four decades for the US intelligence agencies in terms of accumulation of power – and a huge victory for the whistleblower Edward Snowden. Last night at midnight the National Security Agency lost key powers, primarily the bulk collection of phone data. These powers expired for the mammoth surveillance agency because of miscalculations on the part of a complacent Republican leadership out of touch with the mood in America.

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Later this week Congress as a whole is expected to pass legislation – a USA Freedom Act – that will enshrine some of these changes, most importantly the end to the collection of the phone records of millions of Americans, first disclosed in the Guardian almost exactly two years ago in documents leaked by Snowden.

The last time US intelligence agencies suffered a comparable setback was in the 1970s, when the late senator Frank Church led reform in the wake of revelations of spying on civil rights leaders, anti-war protesters, trade unionists, journalists and others. In the four decades since, the intelligence services have been accumulating powers on a scale that even Church could not have envisaged. Changes in technology have made this possible, but so has a lack of transparency and political and legal oversight, with the intelligence agencies left to grow in secret. US reaction to the 9/11 attacks accelerated the process.

The bulk collection of phone data began after 9/11 in secret and was later authorised under section 215 of the US Patriot Act. The process was conducted by a secret court. And it all remained secret until Snowden revealed it in June 2013.

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The Snowden disclosures united both liberals in the Democratic party and members of the libertarian wing of the Republican party, such as the Kentucky senator Rand Paul, who led the opposition in the Senate to the extension of NSA powers. Older, establishment hawks in the Republican party – such as Mitch McConnell, the Senate leader, and senator John McCain – arguing in favour of extension, found themselves squeezed and out of touch.

The importance of the loss of bulk phone collection is not just its demise. The key point is that the intelligence services, after decades left to their own devices, have been exposed to publicity. The idea that their activities can be left free from at least some degree of transparency and oversight is no longer tenable.

Snowden and backers such as Aclu – the American Civil Liberties Union – welcome the end of the programme but stress that it is only a minimal change. The intelligence agencies still possess a vast array of surveillance tools, and they are unlikely to give them up any time soon, if ever.

Aclu instead is pushing for a more realistic outcome, increased transparency and serious political and legal oversight.

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Yesterday the head of the CIA, John Brennan, said the surveillance programmes were “important to American lives”. But Aclu has documented how there is not even a single case in which the intelligence services have demonstrated that mass surveillance has stopped a terrorist attack.

There is no argument about the need for targeted surveillance. The argument is about mass surveillance, and where the balance should be drawn between security and privacy.

Snowden, interviewed by the Guardian in Moscow a fortnight ago, was unwilling to get too excited by the recent federal court decision ruling bulk collection illegal and the pending legislative changes, pointing out all the other programmes that still needed to be addressed.

Almost exactly two years ago, in a hotel in Hong Kong, Snowden said he was leaking the documents because he wanted to start a debate about the powers of the intelligence agencies. He can take satisfaction from the fact that he has succeeded in that. However, the worldwide discussion has turned out to be much bigger than he ever envisaged, and it is not over yet.