FILE - In this Nov. 30, 2016, file photo, Customs and Border Protection U.S. Border Patrol Chief Mark Morgan listens as he testifies during a hearing of the Senate Committee on Homeland Security and Governmental Affairs on Capitol Hill in Washington. Morgan said he was asked to leave and decided to resign on Jan. 26, 2017, rather than fight the request, according to a U.S. official who was on brief video conference in which Morgan informed senior agents of the change. (AP Photo/Alex Brandon)

FILE - In this Nov. 30, 2016, file photo, Customs and Border Protection U.S. Border Patrol Chief Mark Morgan listens as he testifies during a hearing of the Senate Committee on Homeland Security and Governmental Affairs on Capitol Hill in Washington. Morgan said he was asked to leave and decided to resign on Jan. 26, 2017, rather than fight the request, according to a U.S. official who was on brief video conference in which Morgan informed senior agents of the change. (AP Photo/Alex Brandon)

WASHINGTON (AP) — The man charged with protecting America’s borders was ousted Thursday, one day after President Donald Trump announced ambitious plans to build a massive wall at the Mexican border and bolster the ranks of the Border Patrol.

Border Patrol Chief Mark Morgan said he was asked to leave and decided to resign rather than fight the request, according to a U.S. official with knowledge of the brief video conference in which Morgan informed senior agents of the change.

The official spoke on condition of anonymity because the discussion was not intended to be made public.

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The forced resignation leaves Trump with a leadership gap but also gives him a chance to start fresh with a Border Patrol chief of his own choosing.

Border security and a “big beautiful wall” paid for by the Mexican government were centerpieces of Trump’s immigration platform during his presidential campaign. Mexico has repeatedly said it won’t pay for a barrier, and Mexican president Enrique Pena Nieto on Thursday cancelled a meeting with Trump.

Morgan was named to the post in June and took office in October. The former FBI agent briefly led the internal affairs department at the Border Patrol’s parent agency before heading the agency of roughly 20,000 agents.

Gil Kerlikowske, who hired Morgan when he was commissioner of U.S. Customs and Border Protection, said he wasn’t surprised about the leadership change and added: “I have nothing but good and positive things to say about him.”

From the start, Morgan clashed with the Border Patrol’s union, which had advocated for an insider to lead the agency and sharply criticized Morgan at almost every turn.

The union was incensed when Morgan, at a Senate hearing Dec. 1, said he supported a comprehensive immigration overhaul, which is often interpreted to include a path to citizenship for people who are in the country illegally. Morgan clarified his remarks in a note to Border Patrol staff the following week, saying he does not support a “blanket amnesty.”

The union, the National Border Patrol Council, has had a strong relationship with Trump. It was so enamored with his stance on security that it took the unusual step of endorsing him during the campaign. The endorsement was a leadership decision and not decided on by a unionwide vote. Union chief Brandon Judd was part of Trump’s transition team.

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“He didn’t know the job to begin with,” Judd said in an interview with the AP last week. “He had to go on a tour of all the Border Patrol sectors to get an understanding. We needed a chief to hit the ground running.”

When Trump visited the Department of Homeland Security on Wednesday, he singled out Judd while pointedly avoiding mention of Morgan. Morgan was not at the event.

Despite pressure from the union, many agency officials said Morgan appeared to embrace the job. Less than a week ago, the first message on his new Twitter account read, “Chief Morgan here — excited to use this account to share the latest news and events of the #BorderPatrol with followers.”

Deputy Chief Carla Provost, who has been with the agency since 1995, will be the agency’s acting chief, according to an internal memo to agents in the Border Patrol’s Tucson office that was obtained by the AP.

Morgan’s ouster occurred amid a shakeup across the agencies of government.

The State Department on Thursday said several senior management officials as well as a top arms control diplomat would be leaving. All had submitted resignations before Trump became president as required of officials holding president-appointed jobs. But they weren’t required to leave the foreign service.

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Spagat reported from San Diego.

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Follow Elliot Spagat on Twitter at www.twitter.com/elliotspagat and Alicia A. Caldwell at www.twitter.com/acaldwellap