Acting Defense Secretary Patrick Shanahan sent a memo to staffers Tuesday reminding them that their mission is "apolitical."

The notice was sent 10 days after the Navy acknowledged that a request was made to hide the USS John S. McCain during President Donald Trump's recent state visit to Japan.

"Our mission, to protect and defend the nation, is apolitical," Shanahan wrote in the memo.

"DOD has a long-standing policy of encouraging military personnel to carry out the obligations of citizenship, which includes permitting certain political activities. However, our policy and tradition also clearly limit active duty members from engaging in partisan political activities or actions that could appear to imply DOD sponsorship, approval, or endorsement of a political candidate, campaign, or cause."

Trump visited U.S. troops during a trip to Japan on May 28. Earlier this month, Shanahan confirmed details about a Navy email that said the White House military office wanted the USS John McCain kept “out of sight” during Trump's visit.

The internal Navy email, first reported by The Wall Street Journal, triggered a storm of outrage when it came to light late last month.

Trump was a fierce critic of the ship's namesake while the Arizona senator was alive, and he has continued to criticize him even after the Republican lawmaker died more than nine months ago.

The president has said he knew nothing about the request, but added that “somebody did it because they thought I didn’t like him, OK? And they were well-meaning, I will say.”

In the days after the request to move the ship came to light, Shanahan has forcefully spoken out against any attempts to politicize the military.

“Our business is to run military operations and not become politicized,” Shanahan said May 31. “Our job is to run the military,” he added.

“I would not have moved the ship," Shanahan said.

Shanahan has also said he's not planning to seek an investigation by the Pentagon’s internal watchdog into the matter “because there was nothing carried out” by the Navy.