Jim Mattis on Monday formally stepped down as defense secretary, two months ahead of his announced resignation date, with a farewell memorandum to employees that quotes Abraham Lincoln.

“Let nothing which is transpiring, change, hinder, or delay your military movements, or plans,” Mattis, citing the former president, wrote to all Defense Department workers.

“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. Our Department is proven to be at its best when the times are most difficult. So keep the faith in our country and hold fast, alongside our allies, aligned against our foes.

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

A farewell message from Defense Secretary Jim Mattis, who departs his office this evening. pic.twitter.com/Wiix70SBC2 — Dan Lamothe (@DanLamothe) December 31, 2018

Mattis submitted his resignation this month after President Donald Trump abruptly decided to withdraw U.S. troops from Syria, disregarding his defense secretary’s advice that the move could worsen problems in the region.

Mattis, in a resignation letter seen as a rebuke of Trump’s foreign policy, set a departure date of Feb. 28. “Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position,” Mattis wrote.

Trump, annoyed, forced the Pentagon chief out early. Patrick Shanahan will become the acting defense secretary.

Mattis, known as “Mad Dog Mattis” during his time as a Marine, served more than four decades in the military and headed the U.S. Central Command from 2010 to 2013. He backed Trump on strengthening the military and ramping up combat readiness.

Shanahan, as deputy defense secretary, was focused on internal Pentagon reform and the creation of Trump’s promised Space Force, according to Reuters. Shanahan “has a deep-seated understanding of military operations, and global security affairs, and importantly, has the breadth of large-scale business management experience that will enable him to effectively oversee the Defense Department,” Reuters reported, citing a senior administration official.