By ROBERT BURNS and LOLITA C. BALDOR

WASHINGTON (AP) — Pres­i­dent Don­ald Trump fired Defense Sec­re­tary Mark Esper on Mon­day, an unprece­dent­ed move by a pres­i­dent strug­gling to accept elec­tion defeat and angry at a Pen­ta­gon leader he believes wasn’t loy­al enough.

The deci­sion, which could unset­tle inter­na­tion­al allies and Pen­ta­gon lead­er­ship, injects anoth­er ele­ment of uncer­tain­ty to a rocky tran­si­tion peri­od as Joe Biden pre­pares to assume the presidency.

Pres­i­dents who win reelec­tion often replace Cab­i­net mem­bers, includ­ing the sec­re­tary of defense, but los­ing pres­i­dents have kept their Pen­ta­gon chiefs in place until Inau­gu­ra­tion Day to pre­serve sta­bil­i­ty in the name of nation­al security.

Trump announced the news in a tweet, say­ing that “effec­tive imme­di­ate­ly” Christo­pher Miller, the direc­tor of the Nation­al Coun­tert­er­ror­ism Cen­ter, will serve as act­ing sec­re­tary, side­step­ping the department’s No.2‑ranking offi­cial, Deputy Defense Sec­re­tary David Norquist.

“Chris will do a GREAT job!” Trump tweet­ed. “Mark Esper has been ter­mi­nat­ed. I would like to thank him for his service.”

Trump’s abrupt move to dump Esper trig­gers ques­tions about what the pres­i­dent may try to do in the next few months before he leaves office, includ­ing adjust­ments in the pres­ence of troops over­seas or oth­er nation­al secu­ri­ty changes.

Biden has not said who he would appoint as defense chief, but is wide­ly rumored to be con­sid­er­ing nam­ing the first woman to the post — Michele Flournoy. Flournoy has served mul­ti­ple times in the Pen­ta­gon, start­ing in the 1990s and most recent­ly as the under­sec­re­tary of defense for pol­i­cy from 2009 to 2012. She is well known on Capi­tol Hill as a mod­er­ate Demo­c­rat and is regard­ed among U.S. allies and part­ners as a steady hand who favors strong U.S. mil­i­tary coop­er­a­tion abroad.

Miller has most recent­ly served as the direc­tor of the Nation­al Coun­tert­er­ror­ism Cen­ter and before that was a deputy assis­tant Defense Sec­re­tary and top advis­er to Trump on coun­tert­er­ror­ism issues. He has a long back­ground with the mil­i­tary, hav­ing served as an enlist­ed infantry­man in the Army Reserves and after that as a spe­cial forces offi­cer. He also served in the wars in Afghanistan and Iraq. After his retire­ment from the mil­i­tary, Miller worked as a defense contractor.

Esper’s strained rela­tion­ship with Trump came close to col­lapse last sum­mer dur­ing civ­il unrest that trig­gered a debate with­in the admin­is­tra­tion over the prop­er role of the mil­i­tary in com­bat­ing domes­tic unrest. Esper’s oppo­si­tion to using active duty troops to help quell protests in Wash­ing­ton, D.C., infu­ri­at­ed Trump, and led to wide spec­u­la­tion that the defense chief was pre­pared to quit if faced with such an issue again.

Dur­ing his rough­ly 16-month tenure, Esper gen­er­al­ly sup­port­ed Trump’s poli­cies but more recent­ly he was wide­ly expect­ed to quit or be oust­ed if Trump won reelection.

Pres­i­dents his­tor­i­cal­ly have put a high pri­or­i­ty on sta­bil­i­ty at the Pen­ta­gon dur­ing polit­i­cal tran­si­tions. Since the cre­ation of the Defense Depart­ment and the posi­tion of defense sec­re­tary in 1947, the only three pres­i­dents to lose elec­tion for a sec­ond term — Ger­ald Ford, Jim­my Carter and George H.W. Bush — all kept their sec­re­tary of defense in place until Inau­gu­ra­tion Day.

Esper, who was the offi­cial suc­ces­sor to for­mer Marine Gen. James Mat­tis, rou­tine­ly empha­sized the impor­tance of keep­ing the mil­i­tary and the Defense Depart­ment out of pol­i­tics. But it proved to be an uphill strug­gle as Trump alter­nate­ly praised what he called “his gen­er­als” and den­i­grat­ed top Pen­ta­gon lead­ers as war-mon­gers devot­ed to drum­ming up busi­ness for the defense industry.

Trump soured on his first defense sec­re­tary, Mat­tis, who resigned in Decem­ber 2018 over Trump’s abrupt deci­sion — lat­er rescind­ed — to pull all U.S. troops out of Syr­ia, and then on Esper. The splits reflect­ed Trump’s fun­da­men­tal­ly dif­fer­ent views on America’s place in the world, the val­ue of inter­na­tion­al defense alliances and the impor­tance of shield­ing the mil­i­tary from domes­tic par­ti­san politics.

Dur­ing Trump’s tenure, the Pen­ta­gon has often at the cen­ter of the tumult, caught in a per­sis­tent and errat­ic debate over the use of Amer­i­can forces at war in Iraq, Syr­ia and Afghanistan, and on U.S. soil, at the Mex­i­co bor­der and in cities roiled by civ­il unrest and rocked by the coronavirus.

Esper’s depar­ture has appeared inevitable ever since he pub­licly broke with Trump in June over the president’s push to deploy mil­i­tary troops in the streets of the nation’s cap­i­tal in response to civ­il unrest fol­low­ing the police killing of George Floyd. Esper pub­licly opposed Trump’s threats to invoke the two-cen­turies-old Insur­rec­tion Act, which would allow the pres­i­dent to use active-duty troops in a law enforce­ment role. And Trump was furi­ous when Esper told reporters the Insur­rec­tion Act should be invoked “only in the most urgent and dire of sit­u­a­tions,” and, “We are not in one of those sit­u­a­tions now.”

The June civ­il unrest ini­tial­ly drew Esper into con­tro­ver­sy when he joined a Trump entourage that strolled from the White House to near­by St. John’s Epis­co­pal Church for a pho­to op fea­tur­ing Trump hoist­ing a Bible. Crit­ics con­demned Esper, say­ing he had allowed him­self to be used as a polit­i­cal prop.

Esper said he didn’t know he was head­ing into a pho­to op, but thought he was going to view dam­age at the church and see Nation­al Guard troops in the area. He was accom­pa­nied by Gen. Mark Mil­ley, chair­man of the Joint Chiefs of Staff, who lat­er expressed pub­lic regret at hav­ing been present in uniform.

Trump hint­ed at Esper’s shaky sta­tus in August, mak­ing a snide response to a reporter’s ques­tion about whether he still had con­fi­dence in Esper’s lead­er­ship. “Mark ‘Yesper’? Did you call him ‘Yesper?’” Trump said, in what appeared to be an allu­sion to sug­ges­tions that Esper was a yes man for the pres­i­dent. Asked if he was con­sid­er­ing fir­ing Esper, Trump said, “At some point, that’s what happens.”