The political divorce between former Defense Secretary James Mattis and President Donald Trump was about as ugly as these things can get. Although even Trump’s criticisms of Mattis have been somewhat muted compared to his usual (and colorful) posts on the social media platform Twitter, it’s hard to deny that there’s little love lost between the two. As Mattis laid out in his resignation letter, the two do not see eye-to-eye on the very fundamentals of how American defense of foreign policy should be conducted, and as Mattis pointed out, the sitting president deserves to have a cabinet that are able and willing to effectively carry out their vision for the nation.

Once Mattis’ resignation letter hit the media, offering an eloquent argument in favor of the diplomatic norms established through U.S. foreign policy in past decades and indirectly accusing Trump of losing sight of allies in favor of friendly ties with adversaries, one could argue that the president’s hands were tied.

While Mattis offered to remain at his post for two more months while a successor could be located (as the White House is still on the hunt for a chief of staff), Trump made the decision to remove him from office almost immediately. Whether you support the president or not, one could contend that keeping a senior ranking official in-house after such a public rebuke of the policies he’d be responsible for enacting in the coming months, Trump had little choice but to make a quick reshuffling of personnel.

Now, with the Deputy Secretary of Defense, Patrick Shanahan, bumping up to the top spot, the question remains: who will become Mattis’ permanent replacement?

Well, from the looks of the pool of candidates… it may well be Shanahan by default. The former Boeing executive boasts no government or military experience, though one could contend that a crash course of a year under James Mattis may have helped close that experiential gap to some degree. By the standards of most historic administrations, Shanahan likely wouldn’t be a serious contender for the permanent SecDef role… but Trump doesn’t seem to have very many other options.

A number of names have been floated in recent months, as rumors of Mattis’ impending departure have been circulating since well before Trump referred to him as “sort of a democrat” in a recent interview.

Gen. Jack Keane, a retired Army general that earned Trump’s approval through his outspoken criticism of the Obama administration was said to be in the running for the job in the first place, but he has publicly denounced Trump’s planned withdrawal from Syria — a policy the Defense Secretary will need to be on board with in order to oversee the removal of U.S. troops. Other conservative front-runners have similarly made statements pointing to Trump’s seemingly political decision to withdraw troops on a timetable that isn’t informed by military strategy… removing them from the candidacy pool as well.

If enacting Trump’s withdrawal plans with vigor is a requirement for the job, Sens. Lindsey Graham, R-South Carolina, and Tom Cotton, R-Arkansas, both previously said to be on Trump’s short list, are also both out of the running. Graham has even joined Democrats in sponsoring a resolution that calls on Trump to reverse course on Syria altogether. Cotton also signed a bipartisan letter addressed the president calling on him to do the same.