Donald Trump says his proposed 76m (250ft) tall "triumphal arch" in Washington DC would be converted to become a "top-grade military complex" housing drones and snipers and storing ammunition.
Writing on his Truth Social platform, the US president said he had agreed to the move following a "strong request" from the military.
He also cited national security concerns but did not mention any specific threats.
It echoes the rationale he cited for the $400m (£298m) ballroom complex being built on the site of the White House's former East Wing that has been demolished to make way for the project.
His post on Sunday marked a significant change in the arch plans that had already been in progress, as the Trump administration had announced earlier this month that it was preparing to break ground in the coming weeks.
The arch was set to feature a towering winged figure, flanked by two eagles, and guarded by four lions, all gilded with gold.
The design would also carry the words "One Nation Under God" and "Liberty and Justice for All" inscribed in gold atop either side.
From its base to the top of the winged figure, it would dwarf the nearby Lincoln Memorial, which is 30m (99ft) tall.
The arch, which Mr Trump plans to build between the memorial and Arlington National Cemetery, is part of his efforts to change some of the capital's sites and leave a lasting imprint.
They include the White House ballroom, renaming and renovating the Kennedy Center, refurbishing the Lincoln Memorial reflecting pool and rebuilding a golf course in East Potomac Park that could significantly cut down the public's access to running and biking paths.
The president wrote on Truth Social on Sunday: "At the strong request of the United States Military, and for National Security purposes, I have agreed to convert the magnificent Triumphal Arch, planned since the Civil War Era many years ago, at the Receptive Circle adjoining the Arlington Memorial Bridge, into a top grade Military Complex/Triumphal Arch, to house, store, and have the rapid ability to use large numbers of drones, plus Snipers, on both the roof and plaza areas, and additionally have and hold large quantities of sniper ammunition in storage.
"There will be no facility like this anywhere in the World."
Aviation experts have raised safety concerns given its proximity to Reagan Washington National Airport.
And military veterans have launched legal action to block the arch from being built, arguing it would spoil the historic sightline between the Lincoln Memorial and Arlington National Cemetery, and it needs to be approved by Congress.
The project has not yet received final approval from the National Capital Area Planning Commission, which greenlit the site and preliminary plans at a meeting in July.
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Asked to comment on the president's latest remarks, a Pentagon spokesperson said the department has "nothing additional to provide beyond the post".
The White House declined to provide additional information.
(c) Sky News 2026: Donald Trump says planned arch would be military complex for drones and snipers
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