Fox News national security correspondent Jennifer Griffin said President Trump's announcement that U.S. troops will be withdrawn from Syria was the "final straw" for Defense Secretary James Mattis, according to sources.

Mattis, a retired four-star Marine general, on Thursday announced his resignation. He will serve in his role until the end of February.

"Mattis believes pulling out of Syria is a betrayal of the Kurds ... and the Syrian Democratic Forces, U.S. allies who U.S. military leaders believe will be slaughtered after the U.S. leaves Syria, as well as refugees who are on the ground there being protected by U.S. forces," Griffin reported.

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She reported that it appears Mattis’ resignation letter had already been written when he went to the White House at around 3:00pm ET on Thursday to discuss the Syrian withdrawal with the president. Fifty copies of his resignation letter already had been printed and were in a folder in his office to be distributed upon his return.

Griffin shared insight on a particularly telling portion of Mattis' letter.

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. 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.

"From the beginning, Mattis was concerned about how President Trump treated U.S. allies, and spent much of his time as defense secretary reassuring those allies after President Trump disrespected them repeatedly and made decisions without consulting with NATO, South Korea and others," Griffin reported.

She noted that Trump often spoke proudly about the former generals in his administration, including former National Security Adviser Michael Flynn, former National Security Adviser H.R. McMaster, former White House Chief of Staff John Kelly and Mattis.

"In a few weeks, all these generals will be gone. And one former Army general -- Mike Flynn -- is now facing jail time."

Watch Griffin's report above.

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