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.

“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.

The new strategy, Mr. Vaddi said, emphasizes “the need to deter Russia, the PRC and North Korea simultaneously,” using the acronym for the People’s Republic of China.

In the past, the likelihood that American adversaries could coordinate nuclear threats to outmaneuver the American nuclear arsenal seemed remote. But the emerging partnership between Russia and China, and the conventional arms North Korea and Iran are providing to Russia for the war in Ukraine have fundamentally changed Washington’s thinking.

Already, Russia and China are conducting military exercises together. Intelligence agencies are trying to determine whether Russia is aiding the North Korean and Iranian missile programs in return.

The new document is a stark reminder that whoever is sworn in next Jan. 20 will confront a changed and far more volatile nuclear landscape than the one that existed just three years ago. President Vladimir V. Putin of Russia has repeatedly threatened the use of nuclear weapons against Ukraine, including during a crisis in October 2022, when Mr. Biden and his aides, looking at intercepts of conversations between senior Russian commanders, feared the likelihood of nuclear use might rise to 50 percent or even higher.

Mr. Biden, along with leaders of Germany and Britain, got China and India to make public statements that there was no role for the use of nuclear weapons in Ukraine, and the crisis abated, at least temporarily.

“It was an important moment,” Richard N. Haass, a former senior State Department and National Security Council official for several Republican presidents, and the president emeritus of the Council on Foreign Relations, noted in an interview. “We are dealing with a Russia that is radicalized; the idea that nukes wouldn’t be used in a conventional conflict is not longer a safe assumption.”

The second big change arises from China’s nuclear ambitions. The country’s nuclear expansion is running at an even faster pace than American intelligence officials anticipated two years ago, driven by President Xi Jinping’s determination to scrap the decades-long strategy of maintaining a “minimum deterrent” to reach or exceed the size of Washington’s and Moscow’s arsenals. China’s nuclear complex is now the fastest growing in the world.

Although former President Donald J. Trump confidently predicted that Kim Jong-un, the North Korean leader, would surrender his nuclear weapons after their three in-person meetings, the opposite happened. Mr. Kim has doubled down, and now has more than 60 weapons, officials estimate, and the fuel for many more.

That expansion has changed the nature of the North Korean challenge: When the country possessed just a handful of weapons, it could be deterred by missile defenses. But its expanded arsenal is fast approaching the size of Pakistan’s and Israel’s, and it is large enough that it could, in theory, coordinate threats with Moscow and Beijing.

It was only a matter of time before a fundamentally different nuclear environment began to alter American war plans and strategy, officials say.

“It is our responsibility to see the world as it is, not as we hoped or wished it would be,” Mr. Narang said as he was leaving the Pentagon. “It is possible that we will one day look back and see the quarter-century after the Cold War as nuclear intermission.”

The new challenge is “the real possibility of collaboration and even collusion between our nuclear-armed adversaries,” he said.

So far in the presidential campaign, the new challenges to American nuclear strategy have not been a topic of debate. Mr. Biden, who spent much of his political career as an advocate of nuclear nonproliferation, has never publicly talked in any detail about how he is responding to the challenges of deterring China’s and North Korea’s expanded forces. Nor has Vice President Kamala Harris, now the Democratic Party’s nominee.

At his last news conference in July, just days before he announced he would no longer seek the Democratic nomination for a second term, Mr. Biden acknowledged that he had adopted a policy of seeking ways to interfere in the broader China-Russia partnership.

“Yes, I do, but I’m not prepared to talk about the detail of it in public,” Mr. Biden said. He made no reference to — and was not asked about — how that partnership was altering American nuclear strategy.

Since Harry Truman’s presidency, that strategy has been overwhelmingly focused on the Kremlin’s arsenal. Mr. Biden’s new guidance suggests how quickly that is shifting.

China was mentioned in the last nuclear guidance, issued at the end of the Trump administration, according to an unclassified account provided to Congress in 2020. But that was before the scope of Mr. Xi’s ambitions were understood.

The Biden strategy sharpens that focus to reflect the Pentagon’s estimates that China’s nuclear force would expand to 1,000 by 2030 and 1,500 by 2035, roughly the numbers that the United States and Russia now deploy. In fact, Beijing now appears ahead of that schedule, officials say, and has begun loading nuclear missiles into new silo fields that were spotted by commercial satellites three years ago.

There is another concern about Beijing: It has now halted a short-lived conversation with the United States about improving nuclear safety and security — for example, by agreeing to warn each other of impending missile tests, or setting up hotlines or other means of communication to assure that incidents or accidents do not escalate into nuclear encounters.

One discussion between the two countries took place late last fall, just before Mr. Biden and Mr. Xi met in California, where they sought to repair relations between the two countries. They referred to those talks in a joint statement, but by that time the Chinese had already hinted they were not interested in further discussions, and earlier this summer said the conversations were over. They cited American arms sales to Taiwan, which were underway long before the nuclear safety conversations began.

Mallory Stewart, the assistant secretary for arms control, deterrence and stability at the State Department, said in an interview that the Chinese government was “actively preventing us from having conversations about the risks.”

Instead, she said, Beijing “seems to be taking a page out of Russia’s playbook that, until we address tensions and challenges in our bilateral relationship, they will choose not to continue our arms control, risk reduction and nonproliferation conversations.”

It was in China’s interest, she argued, “to prevent these risks of miscalculation and misunderstanding.”