WikiLeaks on Monday launched a searchable archive containing 1.7 million US State Department documents from 1973-76 that had been officially declassified but were not easily accessible to the public.
The "Public Library of US Diplomacy" brings together the archived memos -- referred to as the "Kissinger Cables" after then secretary of state Henry Kissinger -- and the 250,000 cables leaked by the anti-secrecy website in 2010.
WikiLeaks founder Julian Assange said that even though the 1973-1976 cables were declassified, they previously could only be accessed through the US National Archives in a non-searchable PDF format.
The cables were "hidden in the borderline between secrecy and complexity," Assange told reporters in Washington via video link from the Ecuadoran embassy in London, where he has been holed up since last summer.
He also said the documents were at risk of being made secret again, citing a 2006 report by a research institute at George Washington University that found some 55,000 government documents had been secretly reclassified.
"Orwell once said that he who controls the present controls the past and he who controls the past controls the future," Assange said. "Our analysis shows that the US administration cannot be trusted with its control of the past."
Assange later added, with characteristic understatement, that "this material we have published today is the single most significant geopolitical publication that has ever existed."
Although the documents have long been in the public domain, their release in a searchable archive has generated headlines internationally, mainly because the release was coordinated with more than a dozen media outlets.
One such outlet, India's Hindu newspaper, cited the cables in a report saying that Rajiv Gandhi, whose family still dominates India's ruling party, may have been a middleman for an arms deal in the 1970s.
Rajiv Gandhi was assassinated in 1991. His Italian-born widow Sonia is now head of the ruling Congress party and their son Rahul is positioned as a prime ministerial candidate before elections scheduled for next year.
"The corruption in the Gandhi political dynasty is well-known all over the world... and it's about time that the Congress Party of India took its sandals off before entering the corridors of power," Assange said.
Another cable has the Vatican in the 1970s dismissing reports of massacres by Chilean dictator Augusto Pinochet as "Communist propaganda."
Assange said other cables point to the US recruitment of informants in opposition parties and labor unions in several countries and the creation of a "torture exemption" for Brazil in order to allow Washington to provide aid to its rightwing military dictatorship.
The archive can be viewed at wikileaks.org/plusd/
The National Archives and Records Administration could not immediately be reached for comment.
WikiLeaks rose to fame in recent years by releasing hundreds of thousands of secret military logs from Afghanistan and Iraq, as well as the trove of classified US diplomatic cables, all leaked by US Army private Bradley Manning.
Manning admitted to leaking the documents in a statement to a military tribunal in February, pleading guilty to charges that could see him jailed for 20 years in hope of avoiding the more serious allegation of "aiding the enemy."
Assange took refuge at the Ecuadoran embassy nine months ago to avoid extradition to Sweden, where he is wanted for questioning over allegations of rape and sexual assault, which he has denied.