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.

U.S. 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. 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.