President Biden approved in March a highly classified nuclear strategic plan for the United States that, for the first time, reorients America’s deterrent strategy to focus on China’s rapid expansion in its nuclear arsenal.

       The shift comes as the Pentagon believes China’s stockpiles will rival the size and diversity of the United States’ and Russia’s over the next decade.

       The White House never announced that Mr. Biden had approved the revised strategy, called the “Nuclear Employment Guidance,” which also newly seeks to prepare the United States for possible coordinated nuclear challenges from China, Russia and North Korea. The document, updated every four years or so, is so highly classified that there are no electronic copies, only a small number of hard copies distributed to a few national security officials and Pentagon commanders.

       But in recent speeches, two senior administration officials were allowed to allude to the change — in carefully constrained, single sentences — ahead of a more detailed, unclassified notification to Congress expected before Mr. Biden leaves office.

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       “The president recently issued updated nuclear-weapons employment guidance to account for multiple nuclear-armed adversaries,” Vipin Narang, an M.I.T. nuclear strategist who served in the Pentagon, said earlier this month before returning to academia. “And in particular,” he added, the weapons guidance accounted for “the significant increase in the size and diversity” of China’s nuclear arsenal.

       In June, the National Security Council’s senior director for arms control and nonproliferation, Pranay Vaddi, also referred to the document, the first to examine in detail whether the United States is prepared to respond to nuclear crises that break out simultaneously or sequentially, with a combination of nuclear and nonnuclear weapons.

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