How this year's military intelligence leaks could damage US security

It was huge, expensive and top secret.

In the early 1970s the CIA built a gigantic ship called the Hughes Glomar Explorer to lift a sunken Soviet submarine from the bottom of the Pacific Ocean, according to a declassified history by the US intelligence agency.

But the elaborately woven CIA cover story - that the ship was built by Howard Hughes to mine manganese nodules from the ocean depths - began to unravel with a February 1975 Los Angeles Times story, eventually forcing the agency to abandon the project.

The court appearance on Wednesday by Jack Teixeira, a 21-year-old member of the US Air National Guard accused of posting top secret military intelligence records online, has revived questions about whether leaks damage US security in cases less clear-cut than the Hughes Glomar Explorer.

Proving that a leak, whether a single data point or a trove of documents, has harmed the US government is difficult given that internal assessments are themselves kept secret, but analysts of government secrecy said the damage can be dramatic.

"There is a potential … for great damage because many of the most valuable intelligence methods are quite fragile," said Steven Aftergood of the Federation of American Scientists.

"Once their existence is known, they can be evaded or spoofed and so their intelligence value can evaporate," he added, referring to a target taking steps to avoid espionage or exploiting a channel to provide false information.

"Individuals can be placed at significant risk of imprisonment or death," he added.

Mark Zaid, a Washington-based national security attorney, described four types of potential harm.

These include disclosure of the information itself (such as troop locations); the source or method of collection (which can endanger the individual or the stream of information); the mere fact of US interest (which may help adversaries identify and exploit US trigger points); and public disclosure (which can embarrass or provoke other nations, including allies).

There is often diplomatic fallout.

Mexico's president on Tuesday accused the Pentagon of spying after the Washington Post reported on apparent tensions between Mexico's army and navy and said he would begin classifying information from the armed forces to protect national security.

The release of US diplomatic and military documents on Wikileaks starting in 2010 contributed to two US ambassadors losing their assignments.

In 2011, the US ambassador to Mexico resigned after his criticism of Mexican authorities for a lack of coordination against drug cartel leaders emerged and Ecuador expelled the US envoy for cables on suspected police corruption.

It is virtually impossible for outsiders to make a complete appraisal of the damage from leaks because internal assessments are themselves classified to avoid further disclosures.

"The damage assessment itself would likely reveal additional classified information," such as how long a source was providing information and whether what was conveyed, say about military deployments, might have caused a battlefield defeat, Zaid said.

Another complicating factor is that officials can muddy the waters by minimizing the significance of leaks or playing it up, perhaps seeking a public relations benefit by pretending that no harm was done or to make a stronger case for punishing leakers.

In the case of the Hughes Glomar Explorer, which was built at a cost of hundreds of millions of dollars and only recovered part of the Soviet submarine, once its cover was blown it was of no use to the CIA.

The ship was eventually put into private use for deep-water oil drilling and, in 2015, slated to be scrapped.