Milley had called his Chinese counterpart to reassure him that Trump would not attack.

Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, called his Chinese counterpart in the final months of the Trump administration to reassure him that Donald J. Trump had no plans to attack China and that the United States was not collapsing, according to “Peril,” a recently released book by the Washington Post reporters Bob Woodward and Robert Costa.

“Things may look unsteady,” the chairman, Gen. Mark A. Milley, told Gen. Li Zuocheng of China on Jan. 8, two days after Mr. Trump’s supporters stormed the Capitol. “But that’s the nature of democracy, General Li. We are 100 percent steady. Everything’s fine. But democracy can be sloppy sometimes.”

Yet despite his assurances, General Milley was so concerned about Mr. Trump that later that day he convened a meeting with top commanders to remind them that the procedures for launching a nuclear weapon called for his involvement in such a decision.

In the days leading up to the 2020 election, the book reveals, American intelligence showed that the Chinese believed that Mr. Trump planned to launch a military strike to create an international crisis that he could claim to solve as a last-ditch effort to beat Joseph R. Biden Jr.

General Milley, who had become increasingly concerned about the potential for one misread move to set off combat between the world superpowers, first called General Li around that time on a secret backchannel. He wanted to assure General Li and President Xi Jinping that the United States was not planning to attack China.

On the Jan. 8 call, General Li suggested that Chinese leaders feared that the United States government was unstable. He pressed General Milley over the course of an hour and a half about whether the military was going to take action.

General Li feared that Mr. Trump might be trying to find a moment that he could seize on to remain in power, similar to Hitler’s exploitation in 1933 of an arson fire at the German Reichstag to help institute emergency powers, the book said.

Some Republicans have expressed anger about General Milley’s calls to China, but President Biden said earlier this month that he had “great confidence” in him.