Rupert Cornwell: After 9/11, it was always going to be impossible to keep secrets

The massive leak of US diplomatic cables by the Wikileaks organisation is in part an unintended consequence of a decision to step up data-sharing between government agencies to prevent a repeat of the September 2001 terrorist attacks.

It quickly emerged from postmortems on 9/11 that vital information that might have thwarted the attacks did not reach the right people because of barriers between departments and bureaucratic turf wars, not least between the CIA and the FBI.

Accordingly, the Bush administration ordered a wider pooling of information, and a sweeping expansion and reorganisation of the country's fragmented intelligence and security community. The overhaul also entailed a huge increase, to between 500,000 and 600,000, in the number of people with access to Siprnet – the Secret Internet Protocol Network – used by the Pentagon to transmit classified information.

Among Siprnet's more recent users is the State Department, which, according to The Washington Post, wanted to avoid the expense of setting up a network of its own for diplomatic traffic.

Inevitably, however, broader access increased the risk of leaks – either by accident, or deliberately by someone with a grudge against the system. The latter appears to have happened if, as has been alleged, Wikileaks' source turns out to be Bradley Manning, a 23-year-old army intelligence analyst based in Iraq.

Though only a private, Mr Manning had access to Siprnet and thus to the 250,000-plus cables to and from US diplomatic missions. Recently demoted and reportedly facing early discharge, Mr Manning had reason to be disaffected. As he put it in a message to an online confidant before he was arrested in May 2010, he wanted to show "how the First World exploits the Third".

The challenge now for the authorities is to make the system more secure, but without re-erecting the old barriers. In a first move, the Pentagon announced it had removed a feature from its system that allowed data to be transferred on to removable devices. In future two people must be involved in any transfer from secret to non-secret networks.

Vetting of personnel is also likely to be stepped up. But extra precautions rarely have the desired effect. Blockbuster leaks, from the 1971 publication of the Pentagon Papers, to the post 9/11 disclosure the government was engaged in unauthorised eavesdropping, predate the current furore over Wikileaks. There is no reason to suppose there will not be others, given the huge growth of the national security universe since 2001.