The Pentagon leaks: What documents leaked in the US military's worst intelligence breach in a decade reveal

• The leak of a trove of secret Pentagon documents has the US scrambling to contain the fallout.
• The leaked documents revealed US spying on adversaries and allies alike.

A trove of Pentagon documents were leaked online in recent months, representing the worst intelligence breach for the US military in roughly a decade.

Jack Teixeira, a 21-year-old in the Massachusetts Air National Guard, was arrested and charged in connection with the leak.

The Pentagon said the leak poses a serious risk to US national security, and the Biden administration has scrambled to mitigate the fallout even as more classified documents trickle out and reveal the depths of US spying on both allies and adversaries.

Many of the documents date back to March or earlier and relate to the war in Ukraine. The leak also offered compromising details on the extent to which the US has infiltrated Russia's military and intelligence apparatus.

The classified details and then images of the documents themselves were reportedly first shared with a private group of video gamers organized by Teixeira, before one of the users uploaded some of them to a fan group for a popular YouTuber known as wow_mao.

Insider obtained and reviewed copies of photographs of dozens of the leaked documents. It's unclear precisely how many documents were leaked, which largely appear to be authentic. The Washington Post says it has reviewed roughly 300 documents, though the criminal complaint against Teixeira refers to "dozens" of documents. Teixeira was arrested at his home on Thursday.

Here are some of the biggest takeaways from the documents, based on Insider's own review and other reporting:

The US is pessimistic about the state of the Ukraine war

The US assessed that Ukraine's air defenses are vulnerable and it's facing problems with dwindling supplies of munitions, exposing how dire the needs of Ukraine are against their public and repeated calls for more air defense systems.

An assessment from early February suggested Ukraine would only make "modest territorial gains" in a spring counteroffensive due to issues with recruitment, equipment, and ammunition, per the Post.

The US spies on its friends

It's no secret that US intelligence agencies spy on both friends and foes, but it's still embarrassing and uncomfortable for Washington when it's forced to address this publicly.

The documents revealed the US is spying on Ukrainian President Volodymyr Zelenskyy, and showed that he wanted to launch drone strikes against Russian forces in Russia.

One document depicted a potential path for convincing Israel, a US ally that's been reluctant to get to involved in the war, to provide lethal aid to Ukraine.

The leak also revealed concerns within South Korea over the US's push for it to supply ammunition to Ukraine.

Another document appeared to show that Egypt secretly negotiated a deal to supply Russia with artillery rounds and gunpowder — a revelation that could have a detrimental impact on US-Egypt relations if true.

Though the US and Hungary are NATO allies, Hungarian Prime Minister Viktor Orban dubbed the US as one of his party's "top three adversaries," per the Wall Street Journal.

The US has deep knowledge of the Russian military's plans

The documents showed the US has been able to warn Ukraine about upcoming Russian strikes, down to specific locations and dates.

The US assessments on Russia were largely based on signals intelligence, or intercepted communications. The leak could lead Russia to change the way it communicates, making it harder to predict Moscow's next move.

The US is keeping a watchful eye on China

A number of the leaked documents pertained to China, amid historic tensions between the US and Beijing. One document showed the US is concerned that China could use Ukrainian strikes on targets within Russia as an "opportunity to cast NATO as the aggressor, and may increase its aid to Russia if it deems the attacks were significant," CNN reported.

The documented revealed US concerns that Beijing could provide Moscow with lethal aid if NATO-supplied weapons were used by Ukraine for a major strike in Russia.