After being overruled by the White House, outgoing Secretary of Defense James Mattis signed an order on Monday to commence the withdrawal of the roughly 2,000 American troops fighting ISIS in Syria, CBS News confirmed.

Citing strong rifts with President Trump's foreign policy agenda and his treatment of historic U.S. allies, Mattis announced on Thursday that he would be leaving his post in February. Mr. Trump, however, said Sunday that the Pentagon chief would depart by Jan. 1 and be replaced by his deputy and longtime Boeing executive, Patrick Shanahan.

The White House's abrupt decision to order the full departure of U.S. ground forces in Syria on Wednesday provoked a flurry of criticism from Republicans and Democrats. Sen. Marco Rubio, R-Florida, called it a "colossal" mistake, while Sen. Lindsey Graham, R-South Carolina, said it was "a sign of American weakness."

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Brett McGurk, the special presidential envoy for the Global Coalition to Defeat ISIS, announced his resignation on Friday due to his opposition to the troop withdrawal.

Defending his unexpected, controversial decision, the president proclaimed in a Twitter video, "we have won against ISIS."

American forces have been engaged in the ongoing convoluted war in Syria -- which involves President Bashar Al-Assad's Russian-backed military, moderate rebel groups, Iranian-backed Hezbollah cells and ISIS fighters -- since President Obama's tenure.