When Communist Chinese forces began shelling islands controlled by Taiwan in 1958, the United States rushed to back up its ally with military force — including drawing up plans to carry out nuclear strikes on mainland China, according to an apparently still-classified document that sheds new light on how dangerous that crisis was.

American military leaders pushed for a first-use nuclear strike on China, accepting the risk that the Soviet Union would retaliate in kind on behalf of its ally and millions of people would die, dozens of pages from a classified 1966 study of the confrontation show. The government censored those pages when it declassified the study for public release.

The document was disclosed by Daniel Ellsberg, who leaked a classified history of the Vietnam War, known as the Pentagon Papers, 50 years ago. Mr. Ellsberg said he had copied the top secret study about the Taiwan Strait crisis at the same time but did not disclose it then. He is now highlighting it amid new tensions between the United States and China over Taiwan.

While it has been known in broader strokes that United States officials considered using atomic weapons against mainland China if the crisis escalated, the pages reveal in new detail how aggressive military leaders were in pushing for authority to do so if Communist forces, which had started shelling the so-called offshore islands, intensified their attacks.'

The crisis in 1958 instead ebbed when Mao Zedong’s Communist forces broke off the attacks on the islands, leaving them in the control of Chiang Kai-shek’s nationalist Republic of China forces based on Taiwan. More than six decades later, strategic ambiguity about Taiwan’s status — and about American willingness to use nuclear weapons to defend it — persists.

The previously censored information is significant both historically and now, said Odd Arne Westad, a Yale University historian who specializes in the Cold War and China and who reviewed the pages for The New York Times.

“This confirms, to me at least, that we came closer to the United States using nuclear weapons” during the 1958 crisis “than what I thought before,” he said. “In terms of how the decision-making actually took place, this is a much more illustrative level than what we have seen.”

Drawing parallels to today’s tensions — when China’s own conventional military might has grown far beyond its 1958 ability, and when it has its own nuclear weapons — Mr. Westad said the documents provided fodder to warn of the dangers of an escalating confrontation over Taiwan.