Mark Esper

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President Donald Trump announced on Twitter Monday that he has fired Secretary of Defense Mark Esper, and that Christopher Miller, who serves as director of the National Counterterrorism Center, will become acting secretary "effective immediately."

President Trump on Monday said he fired Secretary of Defense Mark Esper and that Christopher Miller, the director of the National Counterterrorism Center, will take over as acting defense secretary "effective immediately."

President Trump announced the ouster of Defense Secretary Mark T. Esper on Monday, marking the exit of Trump’s fourth Pentagon chief and naming a senior intelligence official to take his place.

“Mark Esper has been terminated. I would like to thank him for his service,” the president said on Twitter.

Trump announced the firing via Twitter. He also announced he was putting Christopher C. Miller, the director of the National Counterterrorism Center, as acting secretary of defense “effective immediately.”

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