The top Democrat on the House Armed Services Committee on Wednesday bashed President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE for the forced early departure of Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE, saying that it puts the country “in a riskier position.”

“We face too many challenges and too complex a threat environment to add to our risk by leaving the country without an experienced national security leader in the seat,” Rep. Adam Smith David (Adam) Adam Smith40 groups call on House panel to investigate Pentagon's use of coronavirus funds When 'Buy American' and common sense collide Overnight Defense: Marine Corps brushes off criticism of Marines' appearance in GOP convention video | US troops injured in collision with Russian vehicle in Syria | Dems ask for probe of Vindman retaliation allegations MORE (D-Wash.) said in a statement.

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Trump on Sunday announced he is removing Mattis, who resigned last Thursday, two months earlier than his scheduled departure at the end of February. The president said Deputy Defense Secretary Patrick Shanahan would serve as acting secretary beginning Jan. 1.

Smith called the hasty transition a mistake by Trump, and he called for an explanation as to why Mattis must leave early.

“The President has no given no reason why Mattis can’t stay until the end of February as he planned,” he wrote.

While Smith believes Shanahan will be able to successfully serve as acting Defense Secretary — with management expertise “valuable and conducive to the operations of the Pentagon” — the lawmaker doubts his long-term abilities as Pentagon chief.

“He does not have the comprehensive understanding of global national security threats that Secretary Mattis does. Throwing him into the role of acting secretary with no notice in this way unnecessarily places the United States in a riskier position,” Smith warns.

Mattis announced his resignation after the White House's announcement that it planned to withdraw U.S. troops from Syria — a move the defense chief strongly opposes.

In his resignation letter, Mattis said his world views did not “align” with Trump’s and that he would depart at the end of February.

He added that “over four decades” of experience has taught him the value of alliances such as NATO and the anti-ISIS coalition and of standing strong against adversaries such as China and Russia.

After days of growing increasingly angry over the letter, Trump on Sunday announced he was pushing Mattis out early.