James Mattis last June. Andrew Harrer | Bloomberg | Getty Images

The unprecedented nature of Defense Secretary Jim Mattis' resignation letter to President Trump raises a host of disturbing questions. The most significant of them arise from Secretary Mattis' suggested differences with his Commander-in-Chief regarding the value of allies — and the dangers of strategic, authoritarian competitors. Read Mattis' words closely and they serve to both define and narrow the range of his possible successors to those who better embrace President Trump's world view. The President will be looking for an individual who will share in his suspicion of allies (who he believes don't carry sufficient defense burden while enjoying unfair trade benefits), and who will be more willing to work with adversaries, particularly Russia. "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," wrote Secretary Mattis. "Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other issues, I believe it is right for me to step down from my position." So who that might that individual be? Until recently, Senator Lindsay Graham was considered a front-runner, but this past week he criticized the President's move on Syria, and called for Congressional hearings on both that withdrawal and the potential troop drawdown in Afghanistan. He has been leading the charge against Saudi Crown Prince Mohammed Bin Salman, again out of step with President Trump.

The last time a cabinet official so publicly resigned from a national security position over differences with the President was in 1980.

At the beginning of the Trump administration, Senator Tom Cotton, a military veteran and defense hawk, was considered a leading candidate for either the Pentagon job or CIA director. As a former Army officer who served in Iraq and Afghanistan, he's got the credentials. However, as a potential presidential candidate with a safe Senate seat, he would likely think twice before entering a besieged administration with two years remaining. Retired Army four-star General Jack Keane, former Army Vice Chief of Staff who was thought to be valued by Trump, also seems to have withdrawn himself from the field, saying on Thursday that he and Mattis both opposed Trump's Syria decision – and that he didn't want the Pentagon post. One name is mentioned more frequently these days is that of Deputy Defense Secretary Patrick Shanahan, a frequent White House visitor and former Boeing executive who has built a sound relationship with President Trump and Vice President Pence. He pushed hard within the Department of Defense for Trump's recently announced Space Force, and as more of a business efficiency-expert than a geopolitical or policy architect, he may turn out to be the safe pick. What's clear is that, given the unusual nature of his departure, Mattis' successor is likely to be very different in background and approach to world affairs. The last time a cabinet official so publicly resigned from a national security position over differences with the President was in 1980. It was then that Cyrus Vance quit the Carter administration as Secretary of State in disagreement with President Jimmy Carter's ultimately botched effort to rescue US hostages in ran. What sets Mattis' resignation apart from that episode is that he doesn't point to a single event, as Vance did, but rather to a larger philosophical difference. He says nothing at all about the president's decision to withdraw all 2,000 US troops from Syria, the move that appears to have triggered the resignation.

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