Mattis sent Trump this letter in protest. Thinks the President is weakening America.

President Donald Trump didn't know his Defense Secretary James Mattis was resigning today.

We know that because he sent him well-thought out letter announcing his resignation.

Mattis said near the end of the letter that he disagreed with the president and that Trump "deserved to have a Defense Secretary whose views are better aligned" with his.

He was very diplomatic in his letter, but made it clear that he didn't think Trump was making the right decisions for the country.

Here were a few key takeaways from his letter:

Mattis doesn't agree with how Trump has been treating American allies.

Secondly, he doesn't think Trump has been resolute in America's resolve against Russia and China.

In short, Mattis thinks that Trump's strategy on two main pillars of American defense are wrong, which is standing up for our American friends and against our enemies.

Mattis won't be leaving right away but will instead be resigning at the end of February in order to make for a smooth transition.

It must be noted, however, that Mattis could have waited to resign, but he chose instead to make it known to the world that he didn't want to be apart of this president's foreign policy agenda any longer.