Defense Secretary James Mattis' duty to serve could not be shaken ... until now: ANALYSIS It was almost impossible to separate the Marine from the man.

The resignation of James Mattis is so consequential because of who he is and what he has stood for -- a man who dedicated his entire life to the service of our country.

Even as a civilian Defense Secretary it was almost impossible to separate the Marine from the man. Mattis adored American troops and was happiest visiting them across the United States and around the world, joking time and time again about being "so happy to be out of Washington [he] could cry."

His decades of service and sacrifice in the military became a template for how he would serve an unpredictable president during a time of deep-seeded division in the country.

Whenever questioned about a potential rift between himself and the president, he was never rattled or partisan, always speaking about a duty to serve the Commander-in-Chief that could not be shaken ... until now.

For a man who spoke extensively about the value of U.S. allies and partnerships around the globe, it would make sense for why this week was the final straw. Despite numerous disagreements with Trump on both substance and style, the withdrawal from Syria was a serious betrayal to America's friends and a gift to its foes.

Sidelined, perhaps Mattis finally thought that his advice and guidance to the president was no longer of value -- and he couldn't be party to a decision in Syria, and now in Afghanistan, so counter to his DNA.

With Mattis on his way out the door, it begs the question to those who could replace him: "If Mattis wasn't going to stay? Why should I?"