The U.S. intelligence chief, James Clapper, said this week that the loss of state secrets as a result of leaks by former National Security Agency analyst Edward Snowden was the worst in American history. Clapper backed up his assertion with dire forecasts about emboldened enemies abroad, but some historians and researchers said the U.S. has struggled with even more devastating intelligence breakdowns over the past century.

Clapper, the director of national intelligence, has said Snowden’s disclosures and the resulting media coverage are giving away blueprints for surveillance programs. “Terrorists and other adversaries of this country are going to school on U.S. intelligence sources, methods and tradecraft,” he told the Senate Intelligence Committee on Wednesday.

At the start of that hearing, Clapper staked a claim he had not previously made in public. Snowden’s leaks, he said, were “the most massive and most damaging theft of intelligence information in our history.”

Historians and researchers said Clapper’s remark ignores the most devastating intelligence loss of the 20th century — the theft of America’s top-secret atomic bomb design by Soviet spies. Others say a trio of Americans who spied for Russia in the 1980s and 1990s — Aldrich Ames, Robert Hanssen and John Walker — caused immense intelligence damage that led to the loss of vital secrets and the deaths of American informants.

The Russian spying operation to steal America’s atomic bomb secrets near the end of World War II gave the Soviet government information that sped up its nuclear weapons research and kicked off an arms race that put the world at risk. The theft’s political and cultural ripples deepened the Cold War, aggravated U.S.-Russian enmity and polarized American politics for more than a decade, said Richard Rhodes, author of a three-volume history on the development of the atomic bomb and its repercussions.

“There is no more dangerous intelligence breach across American history than the passage of the designs of our secret nuclear weapons to our worst enemy at the time,” Rhodes said. “Those spies gave the Russians the knowledge they could use to destroy us.”

Soviet spies penetrated the tightly guarded nuclear research facility at Los Alamos, N.M. Physicists Klaus Fuchs and Theodore Hall passed on key design secrets, according to Fuchs’ arrest records and the FBI’s investigation of Hall. The thefts were abetted by another Soviet spy ring, whose members included Julius and Ethel Rosenberg, executed in 1953 for conspiracy to commit espionage.

“The NSA secrets so far revealed, however painful they may be to the NSA, hardly come up to the standard of Klaus Fuchs and Ted Hall,” said Rhodes, who won a Pulitzer Prize in 1988 for “The Making of the Atomic Bomb.”

Other researchers point to the betrayals of a trio of Russian spies. Walker, a former Navy communications specialist, pleaded guilty in 1985 to leading a spy ring for 17 years that decrypted countless U.S. communications and allowed the Soviets to secretly track the U.S. nuclear submarine fleet. Hanssen, a former FBI agent, pleaded guilty to espionage charges in 2001, accused of betraying U.S. informants and providing the Soviets with information on FBI wiretaps and electronic surveillance. And most costly in human lives, Ames, a former CIA counterintelligence officer, was convicted in 1994 of espionage after giving the Soviets information that reportedly led to the deaths of at least 10 U.S. informants.

Intelligence researcher Pete Earley, who wrote books about Walker and Ames, said both spies outdid Snowden because of the classified operational secrets they divulged and the human lives they cost.

“Ames’ betrayal was quantifiable not only in lives but in the loss of critical intelligence sources,” said Peter Earnest, a 35-year CIA veteran who once supervised Ames and is now executive director of the International Spy Museum in Washington.

Earnest said the loss of atomic secrets in the 1940s and the recent intelligence betrayals were signature spy breaches, but so are the Snowden leaks. He cautioned that any comparisons between earlier intelligence breakdowns and the Snowden leaks are tricky — and that it may be years before historians can make an informed assessment.

“These aren’t baseball statistics,” he said. “We know that Snowden has given away our playbook. Is there more to come? What does that mean for us? If anyone has a good guess, it’s Director Clapper.”

Glenn Greenwald, a main recipient of Snowden’s leaks, made a similar assessment about the enormity of the leak, according to the U.S. editor of the British newspaper The Guardian. Editor Janine Gibson said Thursday night that Greenwald told her last summer he thought his Guardian reports using Snowden’s documents could be the “biggest leak in a generation, if not ever.”

Two independent federal panels that reviewed the NSA’s data-collecting programs in recent months chipped away at the importance of Clapper’s signature electronic surveillance program, the bulk collection of telephone data from millions of Americans. Both groups — President Barack Obama’s hand-picked Review Group on Intelligence and Communications Technologies and the semi-independent Privacy and Civil Liberties Oversight Board — concluded that the massive daily intake of phone records was not an essential counterterrorism tool.

Despite those doubts, Clapper’s agency defends his characterization of Snowden’s leaks as the most damaging in American history.

DNI spokesman Shawn Turner told The Associated Press that “when we consider the damage that has already occurred, much of which we can’t discuss in great detail publicly, and the potential damage that will result from continued disclosure of the stolen documents, there’s clearly no comparable previous compromise of intelligence information.”

He added: “The effect on our intelligence efforts is likely to be felt for years to come.”