“'We are on the edge,' one Pentagon official said. 'This is unbelievable.'”

Legislators and some within the Pentagon said they were shaken by Mr. Mattis’s departure and what it could mean for the U.S. military and the nation’s broader national-security strategy. “We are on the edge,” one Pentagon official said. “This is unbelievable.”…

“We probably won’t ever know the true extent of bad ideas he knocked down or slow-rolled,” Christopher Preble, vice president for defense and foreign policy studies at the Cato Institute, said. “I do believe his value to the administration, and, dare I say, the country, was on those occasions when he, by his mere tone, conveyed a sense of competence and good judgment.”

At times, Mr. Mattis didn’t appear to be a part of key military-related decisions. He was on vacation when the president tweeted in July 2017 the end of allowing transgender troops in the military. Officials said Mr. Mattis learned about the president’s decision to suspend major military exercises on the Korean Peninsula after Mr. Trump told North Korean leader Kim Jong Un during their June summit in Singapore. The military has had to adjust its exercises and readiness in the region ever since.