New Acting Chief: Pentagon 'Focused on Safeguarding Our Nation'

Acting U.S. Defense Secretary Pat Shanahan said Tuesday the department "remains focused on safeguarding our nation," as he officially took over the post left by the resignation of Jim Mattis.

"I now look forward to working with President Trump to carry out his vision alongside strong leaders including the service secretaries, the Joints Chiefs of Staff, the combatant commanders, and senior personnel in the Office of the Secretary of Defense," Shanahan said in a statement.

Unlike Mattis, who came to the Pentagon as a revered former Marine general who served in Afghanistan, Shanahan does not have any military experience. Shanahan came to the Defense Department in 2017 from aviation giant Boeing, where he spent more than 30 years overseeing both civilian and military related programs.

Mattis resigned on December 20 following a White House meeting with Trump during which the two men disagreed over the president's decision to withdraw all U.S. troops from Syria, where they have been helping in the fight against the Islamic State terror group.

"Our Department's leadership, civilian and military, remains in the best possible hands," Mattis wrote in his official farewell message Monday, his last day on the job.

"I am confident that each of you remains undistracted from our sworn mission to support and defend the Constitution while protecting our way of life," he continued. "So keep the faith in our country and hold fast, alongside our allies, aligned against our foes."

While it was not the first time the two men disagreed on policy, for Mattis the Syria decision represented a breaking point.

"You have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects," Mattis wrote at the time, adding he would stay on until the end of February 2019 to allow time for a successor to be found and so that he could represent the U.S. at a NATO Defense Ministerial meeting.

Mattis also warned the president that the United States "must be resolute and unambiguous in our approach to countries whose strategic interests are increasingly in tension with ours," naming both China and Russia.

And he further warned that the United States could not afford to alienate allies.

"Our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnership."

Three days later, Trump announced via Twitter that Mattis would be leaving at the end of the year.

According to Pentagon officials, Mattis' departure, at his own request, would not be marked by any of the fanfare normally seen to pay respect to an outgoing defense secretary.

Instead, the handover of authority from Mattis to Acting Defense Secretary Shanahan was to be carried out through a phone call, alerting all relevant government agencies to the change in command.

Shortly after the Pentagon released Mattis' farewell message, the Trump took to Twitter Monday.

"I am the only person in America who could say that, "I'm bringing our great troops back home, with victory," and get BAD press," he wrote.

Mattis began his last message as secretary of defense by quoting from a telegram U.S. President Abraham Lincoln sent to Gen. Ulysses Grant in 1865, a little more than two months before the end of the U.S. Civil War.

"Let nothing which is transpiring, change, hinder or delay your military movements, or plans," it read.