President Trump announced via Twitter today that he is replacing Secretary of Defense (SecDef) James Mattis as of January 1, two months before Mattis said he would depart in his resignation letter last week. Deputy Secretary of Defense (DepSecDef) Patrick Shanahan will become Acting SecDef that day. Shahanan is well known in national security space policy circles as the point man at DOD on creating a Space Force.

Mattis sent his resignation letter to Trump on Thursday after Trump announced he was withdrawing U.S. troops from Syria, a strategic and policy position Mattis does not support. He said he would stay until February 28 to allow an orderly transition.

Trump upended that plan this afternoon, tweeting that Shanahan will become acting SecDef on January 1.

I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019. Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great! — Donald J. Trump (@realDonaldTrump) December 23, 2018

A permanent replacement will require Senate confirmation.

Shanahan spent his career at Boeing before joining DOD. He was senior vice president for Supply Chain & Operations at the time. Before that he was senior vice president of Commercial Airplane Programs; general manager of the 787 Dreamliner program; general manager of Boeing Missile Defense Systems; and vice president and general manager of Boeing Rotorcraft Systems.

Foreign Policy reporter Lara Seligman called the development “just the latest manifestation of the growing influence the world’s largest aerospace company has in Trump’s Pentagon.” She cites Boeing’s wins of three DOD aircraft competitions in the last 6 months “despite massive delays in delivering a new tanker fleet” to the Air Force and “senior Pentagon leaders forcing the Air Force to purchase a new version of Boeing’s legacy F-15 fighter” as examples.

In the space policy arena, Shanahan has been leading the Pentagon’s response to Trump’s June 2018 directive to restructure how it manages space activities. Part of that is creating a sixth military department, a Department of the Space Force, “separate but equal” to the other services.

Shanahan was already developing DOD’s response to a congressional requirement in section 1601 of the FY2018 National Defense Authorization Act (NDAA) regarding the organizational and management structure for national security space. The “section 1601 report” or “Shanahan report” was released by Vice President Mike Pence, chairman of the National Space Council, on August 9 at a Pentagon event.

That report, coupled with Pence’s speech, envisions a number of organizational changes, one of which was officially announced last week — reestablishing a unified combatant command, U.S. Space Command. The others are creation of a Space Operations Force to support U.S. Space Command; a joint Space Development Agency to ensure U.S. Space Command has cutting-edge warfighting capabilities; and a new position of Assistant Secretary of Defense for Space to stand up and scale up the new U.S. Department of the Space Force.

Congress must agree to creating a new military department and a new position of Assistant Secretary of Defense.

Shanahan has been in charge of developing a proposal to submit to Congress as part of the FY2020 budget request. Defense News reported last week that the draft proposal does not, in fact, call for a separate military department. Instead, it would create the “Space Force” as part of the U.S. Air Force.

It is a significant difference. While it may be more attractive to Congress, which has been debating that very solution for several years but calling it a Space Corps (analogous to the Marine Corps within the Department of the Navy), it is not clear if Trump will be satisfied. He has been quite explicit that he wants a sixth military department, not a sub-unit of the Air Force.

How Shanahan’s ascension to Acting SecDef will impact the Space Force debate is an open question.