Defense Secretary Jim Mattis' resignation, which shocked Washington's national security establishment and rattled America's allies, was sealed in a fateful 18-hour period that saw President Trump resolve to withdraw troops from Syria - alarming Pentagon officials who see America's role in the region as crucial.

For Mattis, the president's order was the final straw, Fox News is told.

General Jack Keane, retired 4 star general, former Vice Chief of Staff of the US Army, the chairman of the Institute for the Study of War and Fox News Senior Strategic Analyst, joined Fox News Radio's Guy Benson and Marie Harf to discuss Mattis resigning and Syria. 

On if he would replace Mattis: I'm not going back into public service and I've made that very clear. (1:43)

On what he thinks of Mattis' leaving because of Trump's Syria decision: Secretary Mattis and I had a conversation about it and there's not a hair of separation in terms of our views and what a strategic mistake that decision truly is. I'm hoping that the President will reconsider because if he doesn't I'm confident he's going to come to regret the decision he's made. (2:20)

On his response to people saying what is so bad about leaving Syria: These 2,200 are there to advise the Syrian Democratic Forces, which are made up of Syrian Kurds and Syrian Arabs. The majority of them are indeed Kurds. They have helped them plan operations and they also join them. They were critical in taking down the ISIS safe haven. Since that time, they have spent several months now trying to deal with the remnants of ISIS, most of whom but not all have fled to the southeastern border between Syria and Iraq. A fierce battle has been taking place there these past few months. We just happened to win that battle this past weekend. All that said there's still a few thousand ISIS fighters there and that job needs to be finished... What the Iranians are going to get out of this is dramatically significant and dangerous to our allies in the area and to our own national interest.  I know for a fact, this is a fact, that Netanyahu is fundamentally opposed to this decision. I know for a fact also that he has called the president himself to tell him.  (4:14)

On Trump's video saying this is what the fallen soldiers want: In my own view of that, what the soldiers want to do is finish the job. Finish the job. Go home. Know that the mission is complete. They lost soldiers as a result of this mission. They don't want to squander their lives with an incomplete mission or a humiliating failure later as a result of ISIS returning and retaking territory. That I think they would feel actually let know by that decision. (12:38)