President Donald Trump announced Sunday in a tweet that Deputy Secretary of Defense Patrick Shanahan will take over as Acting Secretary of Defense after the resignation of Jim Mattis due to Trump’s decision to withdraw U.S. troops from Syria.

The tweet also revealed that Mattis, who was originally scheduled to depart his duties at the end of February, will now be exiting by Jan. 1. The New York Times reports that Trump moved up the date after he realized Mattis’ resignation letter slammed Trump’s lack of cooperation with the United States’ allies and inability to check authoritarian governments. Trump had earlier praised Mattis on Twitter when he announced Mattis’ resignation; according to the Times’ source, although Trump had seen the resignation letter at the time, the president didn’t comprehend that it reflected negatively on him.

Trump evidently grew more displeased with Mattis’ letter over the weekend, posting on Saturday that he “gave [Mattis] a second chance” after Obama “ingloriously” fired him from his post as leader of the United States Central Command. “Allies are very important,” Trump wrote, “but not when they take advantage of U.S.”

Shanahan is a former career Boeing executive, having joined the company in 1986 and worked his way up to senior vice president of supply chain and operations by April 2016. The Times reports that aides say Trump likes him in part because he often tells the president that he is correct to complain about the expense of defense systems.