In a sign of his concerns, the nation’s highest-ranking military officer also gathered commanders to remind them of the safeguards in the nuclear launch procedures.

WASHINGTON — The chairman of the Joint Chiefs of Staff twice 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 in an effort to remain in power and that the United States was not collapsing, according to “Peril,” a new 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 to try to stop the certification of his election loss and in the second of two such calls. “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.

The book also reveals how Vice President Mike Pence struggled more than was publicly known over how to navigate Mr. Trump’s demands that he upend the election certification. Speaking privately to former Vice President Dan Quayle, who oversaw the certification of the 1992 election in which he was on the losing ticket, Mr. Pence appeared open to going along with Mr. Trump’s plan, pushed the false claim that Arizona’s voting results were wrong and asked whether there was any way he could delay certification.