WASHINGTON — The acting defense secretary, Patrick Shanahan, notified Congress on Friday that he intended to shift $1.5 billion that had been designated for the war in Afghanistan and other projects to help pay for work on President Trump’s border wall.

The money from the Pentagon’s other programs will be the last that it moves to help build about 80 miles of fencing and barriers along the southern border, Mr. Shanahan said. The newest shift in funding is in addition to the $1 billion that the Defense Department transferred to wall construction in March from the Army’s personnel budget.

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Mr. Shanahan insisted on Friday that the budget transfer would not affect “readiness,” the military catchall term that refers to the ability of American troops to fight when called up. But some of the funds — officials say $600 million — “did come from money we were under-running or saving or whatever, you know, terminology you want to use, from Afghanistan,” he said.

The Pentagon notified Congress of the funding switch on Friday, one day after White House officials said that Mr. Trump would nominate Mr. Shanahan to formally fill the job left vacant when Jim Mattis resigned as defense secretary in December.