The abrupt move to cut short Mattis’s tenure was announced Sunday by Trump in a tweet. It follows waves of criticism over the president’s abrupt change of course on Syria and Afghanistan, which has fueled a sense of turmoil at the highest level of the president’s national security team.

Mattis announced his resignation on Thursday in a letter underscoring his policy differences with the president but saying he’d stay on the job until Feb. 28. The retired Marine Corps general quit after Trump announced his plans to withdraw U.S. troops from Syria.

The interim replacement for Mattis is a former Boeing Co. executive who joined the administration in 2017, reinforcing ties between the Defense Department and the Chicago-based defense contractor. Shanahan will serve in an acting capacity until the president nominates a permanent replacement who is confirmed by the Senate.

“Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing,” Trump tweeted. “He will be great!”

I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019. Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great!

— Donald J. Trump (@realDonaldTrump) December 23, 2018

In his two-page letter dated Dec. 20, Mattis laid bare his disagreements with the commander-in-chief. He criticized the president’s treatment of longtime U.S. allies and implied that Trump’s approach to strategic rivals Russia and China has been ambiguous.

Be Resolute

“We must be resolute and unambiguous in our approach to those countries whose strategic interests are increasingly in tension with ours,” Mattis wrote. He said it’s clear China and Russia “want to shape a world consistent with their authoritarian model” and said the U.S. must respond by using all of the tools of its power — including those alliances — as a counterweight.

Mattis also said the president was entitled to a defense secretary who shared his views and offered to stay on the job until Trump could nominate a successor who could be confirmed by the Senate.

But Mattis’ departure and Trump’s announcements about withdrawing troops from Syria and Afghanistan touched off criticism from allies abroad and from those within his own Republican Party. They expressed concern it could allow terrorist groups to regroup while allowing a freer hand in Syria to Iran, Russia and forces loyal to Syrian President Bashar Al-Assad.

Trump Defense

Trump defended his actions in a tweet on Saturday, saying he and Mattis has an “interesting relationship” and “allies are very important-but not when they take advantage of U.S.”

Brett McGurk, the U.S. envoy to the global coalition fighting Islamic State and an Obama administration holdover, followed Mattis in resigning. Trump tweeted Saturday that McGurk, “who I do not know,” was supposed to leave in February but “just resigned prior to leaving. Grandstander?”

While Shanahan has pledged to recuse himself from Boeing decisions, the defense secretary makes many decisions with a direct or indirect effect on the No. 2 defense contractor and its competitors.

Late Republican Senator John McCain expressed concern about such conflicts during Shanahan’s confirmation hearing for deputy secretary.

Among the immediate decisions that Shanahan may face is whether to accept delivery of Boeing’s new aerial tanker, despite the aircraft’s unresolved flaws. Mattis once vowed not to accept the plane if it’s flawed.