Rep. Adam Kinzinger Adam Daniel KinzingerFox News reporter defends confirming Atlantic piece despite Trump backlash: 'I feel very confident' GOP lawmaker defends Fox reporter after Trump calls for her firing Lindsey Graham: 'QAnon is bats--- crazy' MORE (R-Ill.), an Air Force veteran, ripped 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 for 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 announcement he will resign in February due to disagreements with the president.

“That’s what happens when you ignore sound military advice,” Kinzinger tweeted.

That’s what happens when you ignore sound military advice. https://t.co/SqtX3zYp4j — Adam Kinzinger (@RepKinzinger) December 20, 2018

Trump and Mattis were known to disagree on a multitude of issues, including Trump’s stances toward NATO and Russia, banning transgender troops from serving in the military, canceling joint U.S.-South Korea military exercises, withdrawing from the Iran nuclear deal and deploying troops to the U.S.-Mexico border.

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The defense secretary acknowledged the disagreements in his letter, saying his views did not match the those of the commander in chief.

“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 in a resignation letter devoid of any praise for Trump.

Mattis's departure has long been expected after accounts of disagreements with Trump became more widespread and the president suggested a staff shakeup was imminent following November’s midterm elections.

However, in his tweets announcing the departure, Trump heaped praise on Mattis.

“During Jim’s tenure, tremendous progress has been made, especially with respect to the purchase of new fighting equipment. 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!” he said.