Washington (CNN) Secretary of Defense James Mattis and the State Department's anti-ISIS envoy, Brett McGurk, bid farewell to their respective departments Monday after parting ways with President Donald Trump's administration over his plans to reduce the American presence in Syria.

Mattis told Pentagon employees to "keep the faith in our country" and McGurk wished his former colleagues well "as they work under extremely difficult circumstances to protect the interests of our great country.

"Our Department is proven to be at its best when the times are most difficult," Mattis wrote in a message to Defense Department employees. "So keep the faith in our country and hold fast, alongside our allies, aligned against our foes."

Mattis also expressed confidence in the department's leadership as his deputy defense secretary, Patrick Shanahan, prepares to take over as acting secretary.

"Our Department's leadership, civilian and military, remains in the best possible hands. 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," his statement read.

Mattis concluded, "It has been my high honor to serve at your side. May God hold you safe in the air, on land, and at sea."

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