Former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE criticized the Trump administration as having "abandoned" American beliefs following 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's Thursday resignation announcement.

"I’ve had disagreements with Secretary Mattis, but we shared the view — long-held by Democrats and Republicans in this nation — that respect for our allies and a commitment to the most important and effective alliances in history made America safer," Biden said on Twitter a few hours after the announcement.

Biden is weighing a run for the Democratic nomination for president in 2020.

"It is clear this administration has abandoned those core American beliefs. Secretary Mattis’ presence and his voice of reason and experience will be missed in the Pentagon and the Situation Room," he continued in a second tweet.

I’ve had disagreements with Secretary Mattis, but we shared the view — long-held by Democrats and Republicans in this nation — that respect for our allies and a commitment to the most important and effective alliances in history made America safer. — Joe Biden (@JoeBiden) December 21, 2018

It is clear this administration has abandoned those core American beliefs. Secretary Mattis’ presence and his voice of reason and experience will be missed in the Pentagon and the Situation Room. — Joe Biden (@JoeBiden) December 21, 2018

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Mattis resigned in a letter to President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE dated Thursday, and is expected to leave the White House in February.

Biden referenced Mattis's letter in his tweet, loosely quoting Mattis about the United States' relationship with allies.

"My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues,” Mattis wrote. “We must do everything possible to advance an international order that is most conducive to our security, prosperity and values, and we are strengthened in this effort by the solidarity of our alliances.”

"Because you have a 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 added.

Biden joined many of Democrats who said they were worried or surprised after the resignation. Sen. Mark Warner Mark Robert WarnerDemocrats call for declassifying election threats after briefing by Trump officials It's time to upgrade benefits Intelligence chief says Congress will get some in-person election security briefings MORE (D-Va.), vice chair of the Senate Intelligence Committee, suggested the Trump administration would be destabilized by the defense secretary's departure, and House Minority Leader Nancy Pelosi (D-Calif.) told reporters: "I’m sad. I’m shaken by it. I had so much respect for him.”

Biden and Mattis share a complicated past. Former President Obama is accused of pushing Mattis out of his position in Central Command during in 2013.