WASHINGTON — A trove of leaked Pentagon documents reveals how deeply Russia’s security and intelligence services have been penetrated by the United States, demonstrating Washington’s ability to warn Ukraine about planned strikes and providing an assessment of the strength of Moscow’s war machine.

The documents paint a portrait of a depleted Russian military that is struggling in its war in Ukraine and of a military apparatus that is deeply compromised. They contain daily real-time warnings to American intelligence agencies on the timing of Moscow’s strikes and even its specific targets. Such intelligence has allowed the United States to pass on to Ukraine crucial information on how to defend itself.

The documents lay bare the American assessment of a Ukrainian military that is also in dire straits. The documents, from late February and early March but found on social media sites in recent days, outline critical shortages of air defense munitions and discuss the gains being made by Russian troops around the eastern city of Bakhmut. The intelligence reports show that the United States also appears to be spying on Ukraine’s top military and political leaders, a reflection of Washington’s struggle to get a clear view of Ukraine’s fighting strategies.

The material reinforces an idea that intelligence officials have long acknowledged: The United States has a clearer understanding of Russian military operations than it does of Ukrainian planning. Intelligence collection is often difficult and sometimes wrong, but the trove of documents offers perhaps the most complete picture yet of the inner workings of the largest land war in Europe in decades.

American officials said while the documents offer hints about U.S. methods to collect information on Russian plans, U.S. intelligence agencies do not yet know if any of their sources of information will be cut off as a result of the leak. American officials have conceded they have lost some sources of information since the war began, but the new documents appear to show that America’s understanding of Russian planning remains extensive.

But the leak has the potential to do real damage to Ukraine’s war effort by exposing which Russian agencies the United States knows the most about, giving Moscow a potential opportunity to cut off the sources of information.

The leak has already complicated relations with allied countries and raised doubts about America’s ability to keep its secrets. After reviewing the documents, a senior Western intelligence official said the release of the material was painful and suggested that it could curb intelligence sharing. For various agencies to provide material to each other, the official said, requires trust and assurances that certain sensitive information will be kept secret.

The documents could also hurt diplomatic ties in other ways. The newly reveal intelligence documents also make plain that the U.S. is not just spying on Russia, but also its allies. While that will hardly surprise officials, making such eavesdropping public always hampers relations with key partners, like South Korea, whose help is needed to supply Ukraine with weaponry.

Senior U.S. officials said an inquiry, launched Friday by the Federal Bureau of Investigation, would try to move swiftly to determine the source of the leak. The officials acknowledged that the documents appear to be legitimate intelligence and operational briefs compiled by the Pentagon’s Joint Staff, using reports from the government’s intelligence community, but that at least one had been modified from the original at some later point.

One senior U.S. official called the leak “a massive intelligence breach,” made worse because it lays out to Russia just how deep American intelligence operatives have managed to get into the Russian military apparatus. Officials within the U.S. government with security clearance often receive such documents through daily emails, one official said, and those emails might then be automatically forwarded to other people.

Another senior U.S. official said tracking down the original source of the leak could be difficult because hundreds, if not thousands, of military and other U.S. government officials have the security clearances needed to gain access to the documents. The official said that the Pentagon had instituted procedures in the past few days to “lock down” the distribution of highly sensitive briefing documents. The documents posted online were photographs of folded papers, some with images of a magazine behind them, information that may help investigators.

The documents show that nearly every Russian security service appears penetrated by the United States in some way. For example, one entry, marked top secret, discusses the Russian General Staff’s plans to counter the tanks NATO countries were providing to Ukraine, including creating different “fire zones” and beginning training of Russian soldiers on the vulnerabilities of different allied tanks.

Another entry talks about an information campaign being planned by the GRU, Russia’s military intelligence unit, in Africa trying to shape public opinion against the United States and “promote Russian foreign policy.”

While some of the intelligence briefs offer analysis and broad warnings of Russian plans, others are the kind of actionable information that Ukraine could use to defend itself. One entry talks about the Russian Defense Ministry formulating plans to conduct missile strikes on Ukraine’s forces at specific sites in Odesa and Mykolaiv on March 3, an attack that the U.S. intelligence agencies believed would be designed to destroy a drone storage area, an air defense gun and kill Ukrainian soldiers.

Still another entry discusses a report in February disseminated by Russia’s National Defense Command Center about the “decreased combat capability” of Russia’s forces in Eastern Ukraine.

While the documents were compiled by the Pentagon’s Joint Staff, they contain intelligence from many agencies, including the National Security Agency, the State Department’s Bureau of Intelligence and Research, and the Central Intelligence Agency. Some of the material is labeled as having been collected under the Federal Intelligence Surveillance Act, or F.I.S.A., noting that its further distribution is not allowed without the permission of the attorney general.

One section of the documents is labeled as being the C.I.A. Operations Center Intelligence Update from March 2. That section discusses intelligence on how the Russian ministry of defense had considered steps to counter accusations that it had not been supplying munitions to Wagner group troops in Ukraine “according to a signals intelligence report.”

The documents reveal that American intelligence services are not only spying on the Russians, but are also eavesdropping on important allies.

In the pages posted online, there are at least two discussions about South Korea’s debate about whether to give the U.S. artillery shells for use in Ukraine, violating Seoul’s policy on providing lethal aid. One section of the documents reports that South Korean officials were worried that President Biden would call South Korea’s president pressuring Seoul to deliver the goods.

Another section of the documents, from the C.I.A., is more explicit about how the United States has learned about the South Korean deliberations, noting the information was from “a signals intelligence report.”

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