Retired four-star Gen. Jack Keane said on "The Story" that he understands why Defense Secretary James Mattis felt he needed to resign from his post Thursday. 

President Trump took to Twitter after meeting with Mattis and announced that Mattis would retire "with distinction." Fox News national security correspondent Jennifer Griffin emphasized that Mattis resigned in protest over Trump's foreign policy, one day after Trump ordered a U.S. withdrawal from Syria. 

In a resignation letter to President Trump, Mattis wrote, "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. 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."

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Griffin said it's the first time in 40 years that a cabinet official has resigned over a policy difference with a president. The last such resignation was Secretary of State Cyrus Vance, who quit over Jimmy Carter's failed Iran hostage rescue. 

Keane, the chairman of the Institute for the Study of War, disputed that Americans should be "shaken" by Mattis' departure, as Rep. Nancy Pelosi stated in response. He expressed confidence that Trump will name a "capable successor" and maintain the strength of his national security team. 

He said he spoke to Mattis Thursday and is "of one mind" with the retired top Marine general on the U.S. troop presence in Syria and Afghanistan. When asked by Martha MacCallum whether he would consider becoming the next secretary of defense, Keane said he "has no plans to go back into public service."

Keane said Mattis - who will remain on the job for two more months - did not mention that he would be resigning during their conversation and he's "saddened" to see him depart. 

On Thursday, Keane said Trump was making a "huge strategic mistake" with his decision to pull U.S. forces from Syria. 

"Clearly, we're repeating the Obama mistake of premature withdrawal, which got us ISIS," Keane said.

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