WASHINGTON — The man charged with protecting America’s borders was ousted Thursday, one day after President 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 US 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.

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.

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 US 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 him at almost every turn.