Washington - Defence Secretary James Mattis, who resigned citing irreconcilable policy differences with the president, will be replaced with his Deputy Patrick Shanahan, who will assume the office on January 1 next year, announced US President Donald Trump on Sunday.

“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!” Trump tweeted.

Mattis had resigned on Thursday, a day after Trump shocked the US establishment by pulling out of Syria. In a letter to Trump, Mattis had suggested his worldview, which favours traditional alliances and standing up to “malign actors,” stands at odds with the President’s.

“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. Moments before the Pentagon released Mattis’s letter, Trump tweeted that his defense secretary would be retiring “with distinction, at the end of February.”

“During Jim’s tenure, tremendous progress has been made, especially with respect to the purchase of new fighting equipment,” Trump said. “General Mattis was a great help to me in getting allies and other countries to pay their share of military obligations. A new Secretary of Defense will be named shortly. I greatly thank Jim for his service!”

According to reports, 68-year-old Mattis, a retired United States Marine Corps General, has gone to the White House on Thursday afternoon in a last attempt to convince Trump to keep US troops in Syria. He was rebuffed, and told the president that he was resigning as a result.

Separately on Thursday, there were reports the White House was also planning a sharp cut to troop numbers in Afghanistan. In his extraordinary resignation letter, Mattis told Trump he had “the right to have a Secretary of Defence whose views are better aligned with yours”. Mattis’ resignation letter, a Pentagon spokeswoman said was hand-delivered to the president.

In the letter, Mattis told Trump it was the “right time” for him to step down. “The end date for my tenure is February 28, 2019, a date that should allow sufficient time for a successor to be nominated and confirmed as well as to make sure the Department’s interests are properly articulated and protected at upcoming events to include Congressional posture hearings and the NATO Defense Ministerial meeting in February,” Mattis said.

The Pentagon chief did not mention if he was resigning specifically over the troop withdrawal decision, which has surprised US allies and even Republican lawmakers.

Mattis was a great advocate of strong Indo-US defence relationship and New Delhi’s growing role in the strategic Indo-Pacific amidst China flexing its muscles in the region. Earlier this month, Mattis hosted Defence Minister Nirmala Sitharaman at the Pentagon for talks on India-US defence relationship. 

“The US-India relationship is a natural partnership between the world’s oldest and the world’s largest democracy,” Mattis had said then. He visited India in September for the inaugural India-US 2+2 Dialogue.

In April this year, Mattis appealed to Congress to urgently provide India the national security waiver, saying imposing sanctions on it for buying the S-400 air defence missile system from Russia would only hit the US. Analysts say the withdrawal of troops will please US enemies by clearing the way for Syria’s Bashar al-Assad regime, Russia and Iran. Mattis had warned that removing ground forces from the Middle Eastern country would be a “strategic blunder”.

US President Donald Trump is seen with Deputy Secretary of Defence Patrick Shanahan earlier this year. Shanahan will become U.S. secretary of defence in the new year. (Pablo Martinez Monsivais/Associated Press)

U.S. President Donald Trump on Sunday said he was replacing Defence Secretary Jim Mattis on Jan. 1 with Deputy Defence Secretary Patrick Shanahan in an acting capacity after Mattis resigned over policy differences and offered to stay in his post for two more months.

Mattis’s resignation letter implicitly criticized Trump’s foreign policies and his treatment of military allies after the president’s decision to pull troops out of Syria.

“Because you have a right to a secretary of defence 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 said in his resignation letter, released by the Pentagon.

Trump announced Shanahan’s appointment in a tweet Sunday, calling the former Boeing Co executive “very talented.”

In a shock announcement on Wednesday, Trump said he was withdrawing U.S. troops from Syria, a decision that upended American policy in the region.

Shortly after posting this, he shared details of a “productive call” he had had with President Recep Tayyip Erdogan of Turkey.

Mr Trump said the pair had discussed “our mutual involvement in Syria, & the slow and highly coordinated” withdrawal of US troops from the country.

While not mentioning it in his resignation letter directly, he is understood to oppose the decision and has previously warned that an early withdrawal would be a “strategic blunder”.

US President Donald Trump, who aides said has been frustrated by news coverage of Defence Secretary Jim Mattis’s scathing resignation letter, has abruptly announced that he was removing Mattis two months before his planned departure and installing Patrick Shanahan as acting defence secretary.

Mattis resigned in protest last week after Trump announced the withdrawal of US troops from Syria - a move that Mattis and others on the national security team strongly counselled the president against.

In his Thursday resignation letter, Mattis delivered a sharp rebuke of Trump’s worldview and cast the president’s foreign policy positions as a threat to the nation. Trump has complained to advisers about the brutal media commentary that ensued.

Mattis said in his letter that he would resign his post on February 28, to allow for a smooth transition to the next defence secretary.

But Trump decided to hasten the process, announcing on Sunday on Twitter that Shanahan would replace Mattis imminently.

“I am pleased to announce that our very talented Deputy Secretary of Defence, Patrick Shanahan, will assume the title of Acting Secretary of Defence starting January 1, 2019,” Trump wrote.

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