• A leak of highly classified Pentagon documents has undermined trust among U.S. allies, former U.S. officials and intelligence experts tell CNBC.
• U.S. authorities on Thursday arrested 21-year-old Jack Teixeira, a low-ranking member of the Massachusetts Air National Guard, in connection with the investigation into the leak.
• America's control over its most valuable secrets has been thrust into question by the fallout from the most damaging intelligence leak since Edward Snowden's breach more than a decade ago,

An embarrassing leak of highly classified Pentagon documents has endangered intelligence methods, exposed American strategy and undermined trust among U.S. allies, former defense department officials and intelligence experts tell CNBC.

U.S. authorities on Thursday arrested 21-year-old Jack Teixeira, a low-ranking member of the Massachusetts Air National Guard, in connection with the investigation into the leak.

America's control over its most valuable secrets has been thrust into question amid the fallout from the most damaging intelligence leak since Edward Snowden's breach more than a decade ago.

"It's hard to trust us with your secrets if we can't protect them," said Bill Lynn, a former U.S. Deputy Secretary of Defense under President Barack Obama, who now serves as Chief Executive of Leonardo DRS.

The trove of classified documents, which first appeared on the Discord social media site last month, revealed stunning details about U.S. spying on Russia's war efforts in Ukraine and secret information about Ukraine's combat power, according to NBC News reporting.

"It gives the Russians insight into how we're gathering that information, which puts those sources at risk," Lynn said.

'Devastating' for American allies

The major security breach also contained intelligence gathering on American allies, including South Korea and Israel.

"It's devastating to our allies to see that kind of information being promulgated," Lynn said. "It was shared too widely ... but that's 20-20 hindsight and easy to say now," he added. 

"There is always an intention in intelligence to provide the information to the people who need it so it can be used, and then protecting it from disclosure. Obviously, in this case, we didn't do enough to protect it."

'Sloppy and incompetent'

The nature of the leak — and revelations that some of the documents may have been out for as much as a year before the U.S. defense department caught on — makes the U.S. government look unreliable and incompetent, one former CIA officer told CNBC.

"The fact that a 21-year-old kid had access to this kind of material? Our allies are seeing us as sloppy and incompetent," said Marty Martin, who served several years in the CIA's Senior Intelligence Service, as well as the NSA and the U.S. Army.

The White House responded to questions highlighting this concern, saying that the Pentagon was further restricting access to sensitive information and that an investigation is ongoing.

But Martin — who led the CIA team that tracked down Osama bin Laden and numerous other Al Qaeda operatives — believes the environment that enabled such a breach was the government's own doing.

"In 2002-2003, in the aftermath of 9/11, there was a big push by the DoD, the DNI (Directorate of National Intelligence) and Congress to have all the intelligence data combined," Martin recounted, describing his concern at the time that this would allow far too much access to highly sensitive material.

Many in the U.S. government believed that a lack of inter-agency intelligence sharing prior to the September 11^th attacks led the country to be blindsided and unprepared. The policy was therefore changed to increase information sharing.

'A total disaster'

Martin believes that this approach led to the intelligence breach that the country is now grappling with.

"So all the databases got combined, and now you have some 21-year-old National f---ing Guard guy having access to the CIA's operational secrets. The crown jewels of top-secret intel in Washington became like a little game."

"A guy at the Pentagon who's counting tanks does not need to have access to sensitive counterterrorism information or operations information at the CIA," Martin said. "It's a total disaster."

The consequence for international alliances is serious, Martin added.

"Our allies can't trust us … That's why the Middle East, they're talking to the Chinese. And the Saudis are talking to Iran. Why? There's a void of American leadership."

CNBC has reached out to the Pentagon and White House for comment.

U.S. credibility at risk?

Even with the increase in shared intelligence among U.S. agencies in recent years, observers are still baffled that such a junior employee would have access to CIA reports.

"How can a young ANG (Air National Guard) … have this sort of access? How does this kid have this intel sitting in a ANG base on Cape Cod? How was he able to (1) print them out (2) take them home?" one American defense industry executive told CNBC, speaking anonymously due to professional restrictions.

"Strangely, one of the documents is an internal CIA document — as internal, it would never have been disseminated to other agencies, not even within the intelligence community. How is that out there?" the executive said, referring to the leaked documents.

NBC has not verified the authenticity of the documents. The U.S. government is treating them as authentic but warns that some appear to have been altered. 

The executive questioned how an Air National Guard server would have access to CIA reports.

"I'm just not convinced that an ANG base has access to these types of documents. So I'm wondering if other docs got added to the mix," he said.

Crucially, he said — echoing Martin's concerns — the leak damages U.S. credibility and trust among alliances. He pointed to the leaked files, some of which he had seen, that painted a grim picture of Ukraine's future prospects in battle.

"The key point is really not the leaker," the executive argued. "It's the fact [that] somehow these documents got out there, which basically say, in general, what professionals have known for months: Ukraine will probably not win the war," he said, adding that he works with the Ukrainians.

Kyiv strongly disagrees with such a position, expressing consistent confidence in the ability of its forces to beat the Russians, provided that they keep receiving a steady stream of Western military support.   

"As long as we keep publicly telling everyone that 'Russia is going to lose very soon,' we lose credibility," he argued. "And that's why India, Africa, Mid-East, etc. pursue their own policies and have their own intel networks."