The appropriateness of Milley's call to China's Li depends on what was said and who was in the loop.
According to the Washington Post, "Peril" quotes Milley easing Li's concerns.
"General Li, I want to assure you that the American government is stable and everything is going to be okay," Milley told him Jan. 8, according to the Post. "We are not going to attack or conduct any kinetic operations against you."
By itself, such calls fall within established practice, said historian Richard Kohn at the University of North Carolina.
"I know of no written rules for such contacts other than common sense, traditional practices," Kohn said.
And he listed them.
"Probity, cautiousness, carefulness and avoiding any compromise of American secrets, classified information, or any violations of American policy or orders from the president or secretary of defense."
Joyner, of the Marine Corps University, said the appropriateness of Milley's call hinges on what he said.
"If Milley simply reassured his counterpart, in a room full of other officials that was very much on the record, that the U.S. had no intention of starting a war with China, it's not only acceptable but sound," Joyner said.
But, Joyner added, if it turned out that Milley said he would refuse to obey an order from Trump to launch attacks, that would be "highly inappropriate."