[WASHINGTON] President Donald Trump directed defence secretary James Mattis to depart his administration by Jan 1, two months earlier than planned, and will replace him with the Pentagon's second-ranking official Patrick Shanahan.

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

While Mr Mattis announced his resignation Thursday in a scathing letter underscoring his differences with the president over the role of American leadership and alliances, he said he would stay on until Feb 28 to help provide a bridge to the next defense chief.

Mr Trump initially praised the retired four-star Marine general, but his mood soured after reading the entirety of Mr Mattis' letter, according to an administration official who described Mr Mattis' missive as inappropriate. It fell to Secretary of State Michael Pompeo to tell Mr Mattis on Sunday morning that he had about one week to finish up at the Pentagon and make room for Shanahan, according to the official, who asked not to be identified.

Interim Replacement

The interim replacement for Mr Mattis is Mr Shanahan, a former Boeing executive who joined the administration in 2017, reinforcing ties between the Defense Department and the Chicago-based defense contractor. Mr 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," Mr Trump tweeted. "He will be great!"

Mr Trump wanted Mr Shanahan to get started in the job Jan 1 to let him get to work laying the groundwork for the fiscal year 2020 budget request and to ensure there is no confusion about defence policy given Mr Mattis's opposition to the president's decision to begin withdrawing troops from Syria, according to the administration official.

According to a White House official, Mr Trump wants to streamline the Pentagon to make it more efficient for purchasing and getting better deals and feels Shanahan would be better poised to move that effort forward in the acting role.

Mr Shanahan could eventually be nominated to formally replace Mr Mattis, but there are other potential candidates for the position as well, according to both the administration and White House officials.

Coming after Mr Mattis, a decorated warrior, Mr Shanahan's lack of military service could be an issue for some of the commanders and troops he will lead. But a number of past defence chiefs had no military record. Most recently, Ashton Carter, who was defense secretary under President Barack Obama, was never in the military, although he had a lengthy resume in civilian defense posts and related academic positions.

In his two-page letter dated Dec 20, Mr Mattis criticised the president's treatment of longtime US allies and implied that Mr 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," Mr Mattis wrote. He said it's clear China and Russia "want to shape a world consistent with their authoritarian model" and that the US must respond by using all of the tools of its power - including those alliances - as a counterweight.

Mr 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 Mr Mattis' departure and Mr 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 Defence

Mr Trump defended his actions in a tweet on Saturday, saying he and Mr Mattis have an "interesting relationship" and "allies are very important-but not when they take advantage of US" He also said he should be getting credit for bringing "our young people safely back home" from overseas wars.

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

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

The late Republican Senator John McCain expressed concern about such conflicts during Mr Shanahan's confirmation hearing for deputy secretary.

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

But even more importantly, Mr Shanahan will find himself tasked with successfully bringing home nearly 10,000 troops from Syria and Afghanistan, a shift of geostrategic priorities that seemed unthinkable just a week ago.

BLOOMBERG