On Thursday, US President Donald Trump announced that Defense Secretary James Mattis will depart the Cabinet effective February 28, 2019.

"Because you have the right to have a Secretary of Defense whose views are better aligned with yours," Mattis wrote in his resignation letter, "I believe it is right for me to step down from my position.

Reuters reported shortly after the announcement that Mattis had retired following a meeting with Trump on Thursday in which the two could not agree on several issues, citing a senior White House aide.

Mattis noted that one of his core beliefs was "that our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnerships."

Mattis' resignation was met with disappointment, shock and concern by lawmakers Thursday.

When 80-year-old Leon Panetta, who has served in several public office positions, including the Secretary of Defense, director of the CIA, White House chief of staff, director of the Office of Management and Budget, was interviewed by CNN's Erin Burnett on Thursday, he criticized Trump's inability to take the advice of his advisers.

"He enjoys chaos because he thinks chaos produces attention for him," Panetta pointed out.

"Just read Gen. Mattis resignation letter," Republican Sen. Marco Rubio of Florida tweeted. "It makes it abundantly clear that we are headed towards a series of grave policy errors which will endanger our nation, damage our alliances & empower our adversaries."

Ohio Republican Gov. John Kasich also referred to Mattis' resignation as "chaos," and tweeted that America is "in danger."

Mattis's resignation followed Trump's Wednesday decision to pull all 2,000 troops from Syria beginning immediately, following the latter's declaration of victory over Daesh. 

According to reports, Mattis's departure from the White House was spurred by a phone conversation between Trump and Turkish counterpart Recep Tayyip Erdogan, in which POTUS decided to withdraw troops from Syria, the Associated Press reported, citing two US officials and a Turkish official briefed on the matter.

Sources claim that Trump, who previously entreated with Erdogan to not put US troops at risk, made a U-turn, now siding with Erdogan, while Mattis and other national security officials attempted to point out the error of his ways.