An innocuous-looking memory stick, no longer than a couple of fingernails, came into the hands of a Guardian reporter earlier this year. The device is so small it will hang easily on a keyring. But its contents will send shockwaves through the world's chancelleries and deliver what one official described as "an epic blow" to US diplomacy.

The 1.6 gigabytes of text files on the memory stick ran to millions of words: the contents of more than 250,000 leaked state department cables, sent from, or to, US embassies around the world.

What will emerge in the days and weeks ahead is an unprecedented picture of secret diplomacy as conducted by the planet's sole superpower. There are 251,287 dispatches in all, from more than 250 US embassies and consulates. They reveal how the US deals with both its allies and its enemies – negotiating, pressuring and sometimes brusquely denigrating foreign leaders, all behind the firewalls of ciphers and secrecy classifications that diplomats assume to be secure. The leaked cables range up to the "SECRET NOFORN" level, which means they are meant never to be shown to non-US citizens.

As well as conventional political analyses, some of the cables contain detailed accounts of corruption by foreign regimes, as well as intelligence on undercover arms shipments, human trafficking and sanction-busting efforts by would-be nuclear states such as Iran and Libya. Some are based on interviews with local sources while others are general impressions and briefings written for top state department visitors who may be unfamiliar with local nuances.

Intended to be read by officials in Washington up to the level of the secretary of state, the cables are generally drafted by the ambassador or subordinates. Although their contents are often startling and troubling, the cables are unlikely to gratify conspiracy theorists. They do not contain evidence of assassination plots, CIA bribery or such criminal enterprises as the Iran-Contra scandal in the Reagan years, when anti-Nicaraguan guerrillas were covertly financed.

One reason may be that America's most sensitive "top secret" and above foreign intelligence files cannot be accessed from Siprnet, the defence department network involved.

The US military believes it knows where the leak originated. A soldier, Bradley Manning, 22, has been held in solitary confinement for the last seven months and is facing a court martial in the new year. The former intelligence analyst is charged with unauthorised downloads of classified material while serving on an army base outside Baghdad. He is suspected of taking copies not only of the state department archive, but also of video of an Apache helicopter crew gunning down civilians in Baghdad, and hundreds of thousands of daily war logs from military operations in Afghanistan and Iraq.

It was childishly easy, according to the published chatlog of a conversation Manning had with a fellow-hacker. "I would come in with music on a CD-RW labelled with something like 'Lady Gaga' … erase the music … then write a compressed split file. No one suspected a thing ... [I] listened and lip-synched to Lady Gaga's Telephone while exfiltrating possibly the largest data spillage in American history." He said that he "had unprecedented access to classified networks 14 hours a day 7 days a week for 8+ months".

Manning told his correspondent Adrian Lamo, who subsequently denounced him to the authorities: "Hillary Clinton and several thousand diplomats around the world are going to have a heart attack when they wake up one morning and find an entire repository of classified foreign policy is available, in searchable format, to the public ... Everywhere there's a US post, there's a diplomatic scandal that will be revealed. Worldwide anarchy in CSV format ... It's beautiful, and horrifying."

He added: "Information should be free. It belongs in the public domain."

Manning, according to the chatlogs, says he uploaded the copies to WikiLeaks, the "freedom of information activists" as he called them, led by Australian former hacker Julian Assange.

Assange and his circle apparently decided against immediately making the cables public. Instead they embarked on staged disclosure of the other material – aimed, as they put it on their website, at "maximising political impact".

In April at a Washington press conference the group released the Apache helicopter video, titling it Collateral Murder.

The Guardian's Nick Davies brokered an agreement with Assange to hand over in advance two further sets of military field reports on Iraq and Afghanistan so professional journalists could analyse them. Published earlier this year simultaneously with the New York Times and Der Spiegel in Germany, the analyses revealed that coalition forces killed civilians in previously unreported shootings and handed over prisoners to be tortured.

The revelations shot Assange and WikiLeaks to global prominence but led to angry denunciations from the Pentagon and calls from extreme rightwingers in the US that Assange be arrested or even assassinated. This month Sweden issued an international warrant for Assange, for questioning about alleged sexual assaults. His lawyer says the allegations spring from unprotected but otherwise consensual sex with two women.

WikiLeaks says it is now planning to post a selection of the cables. Meanwhile, a Guardian team of expert writers has been spending months combing through the data. Freedom of information campaigner Heather Brooke obtained a copy of the database through her own contacts and joined the Guardian team. The paper is to publish independently, but simultaneously with the New York Times and Der Spiegel, along with Le Monde in Paris and El País in Madrid. As on previous occasions the Guardian is redacting information likely to cause reprisals against vulnerable individuals.