Defense Secretary Mark Esper has been "terminated," President Trump wrote in a tweet, and will be replaced by Christopher C. Miller, the director of the National Counterterrorism Center.

"Chris will do a GREAT job," Trump tweeted shortly after noon. "Mark Esper has been terminated. I would like to thank him for his service."

Sources say Esper already had a resignation letter ready to go — because Trump threatened to fire him in June over a disagreement about using active duty troops to quell street protests — and had recently updated it.

Esper is the former Army secretary and Raytheon executive who took over the job in July of last year from retired Marine Gen. Jim Mattis. Mattis abruptly resigned over efforts by Trump to quickly reduce troops in Syria.

Esper had the least amount of government and military experience of any defense secretary in memory, and he never had the kind of strong relationship Mattis had with Trump — at least until Trump soured on Mattis as well.

Trump also leaned more on two more-forceful officials: Secretary of State Mike Pompeo and Joint Chiefs Chairman Gen. Mark Milley.

Esper earned the derogatory nickname "Yesper" for seemingly acquiescing or remaining silent over the president's kneejerk moves. Those ranged from reducing U.S. troops in Afghanistan and Syria with little or no deliberation to stopping Pentagon efforts to rename military bases named after Confederate generals.

"He came in with such promise," said one Capitol Hill aide who was not authorized to speak publicly. The aide, who has known Esper for years, called him a "logical, clear thinker" whose tenure as Army secretary was noteworthy for cutting programs deemed no longer necessary. "His performance as secretary of defense was disappointing."

Just a few weeks ago, national security adviser Robert O'Brien said U.S. troops in Afghanistan would be reduced sharply early next year. Milley brushed that aside as "speculation" in an interview with NPR and said deliberations over troop cuts were ongoing.

Esper was nowhere to be found in that discussion, though Esper's supporters said he worked quietly behind the scenes.

"What did he do behind the scenes and what did he do to speak out?" wondered Brad Bowman of the Foundation for Defense of Democracy, who said Trump's efforts to quickly cut troops in Syria and Afghanistan — as well as some 12,000 American forces in Germany — were "fundamental policy mistakes" with seemingly little thought about strategy or repercussions.

Others, though, say Esper made some positive accomplishments during his brief tenure.

"I think he was solid and successful," said Michael O'Hanlon, a defense analyst at the Brookings Institution, pointing to Esper's continued focus on a new defense strategy that singled out China and Russia as threats the Pentagon had to address.

"He handled his relationship with the president about as well as you could have asked any mortal to achieve," said O'Hanlon. "No-drama Esper was what we needed. And it was what we got."

Ironically, Trump had planned to fire Esper four months ago — after Esper opposed the president's move to send active duty troops to Washington, D.C.

Protesters were massing across from the White House, decrying the death of George Floyd at the hands of Minneapolis police. Pentagon officials and others said police and National Guard troops could deal with any situation.

But Trump wanted to send a stronger message. So he ordered the deployment of active troops from bases in North Carolina and New York. Pentagon leaders made sure they stayed outside the city.

Esper explained why.

"The option to use active duty forces in a law enforcement role should only be used as a matter of last resort, and only in the most urgent and dire of situations," Esper told reporters at the Pentagon on June 3, his last meeting with reporters in the briefing room. "We are not in one of those situations now. I do not support invoking the Insurrection Act."

Trump was furious. But a senior official said Trump was talked out of dismissing Esper, with his advisers saying it was not a good idea to fire the defense secretary — his third one — so close to the election.

Even before that, the White House was not happy with Esper, according to Politico, for his supposedly "loose" messaging with the press, including an interview with NPR in January.

The interview took place shortly after a U.S. drone strike killed Iranian military leader Qassem Soleimani, who oversaw his country's militia groups in Iraq that were targeting U.S. troops and officials.

Esper told NPR that the U.S. did not have the authority to target those militias under the 2002 Authorization for Use of Military Force enacted after the Sept. 11 attacks.

Moments after the interview ended, Esper's aides chased down the NPR team and brought them back to the secretary's office

Esper then reversed himself: The administration actually does have that authority under Article 2 of the Constitution, the commander in chief's authority to defend the nation.

By summer, Trump was openly ridiculing his defense secretary. At a White House press briefing in August, one reporter asked about the defense secretary.

"Mark Yesper? Did you call him Yesper?" Trump said. "Oh OK. Some people call him Yesper. No I get along with him. I get along with him fine. He's fine. Yeah no problem."

Asked if he was considering firing Esper, Trump said: "I consider firing everybody. At some point, that's what happens."

After it was clear Trump was ready to fire him, Esper kept a low profile, rarely talking to reporters and going on trips to Africa and India and appearing in recent weeks before friendly think tanks, where few tough questions would be asked.

In those appearances, he focused on the National Defense Strategy, which called for a greater focus on the threats from China and Russia.

"Our primary competitors, China and Russia, are rapidly modernizing their armed forces," Esper told the Atlantic Council last month, "and using their growing strength to ignore international law, violate the sovereignty of states and shift the balance of power in their favor."

As part of that effort, said Brad Bowman, a defense analyst with the Foundation for Defense of Democracies, Esper will be remembered for his efforts to counter those modernizing efforts by China and Russia.

Esper pushed for more weapons research and development in such areas as hypersonics and directed energy. Such "cutting edge technologies," Bowman said, will put the Pentagon "in a much better place."

In the end, said O'Hanlon, there were no major mishandled crises on Esper's watch and the U.S. maintained deterrence in key theaters.

"I think he should be proud of his record," O'Hanlon said, "even if his role was closer to caretaker than a revolutionary."

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