Dozens of reporters have turned in their access badges and exited the Pentagon rather than agree to a new media policy that restricts how journalists operate.
Journalists were asked to sign a pledge to refrain from reporting information not authorised for release by the US Department of War, formerly the Department of Defense.
The US government has described the new rule as "common sense".
News outlets were nearly unanimous in rejecting the rules imposed by Defense Secretary Pete Hegseth.
The order would leave journalists vulnerable to expulsion if they sought to report on information – classified or otherwise – that had not been approved by Mr Hegseth for release.
Only the conservative One America News Network signed on.
Many of the reporters waited to leave the building together when the deadline set by the Defense Department arrived at 4pm on Tuesday, local time.
As the hour approached boxes of documents lined a Pentagon corridor and reporters carried chairs, a copying machine, books and old photos to the parking lot from their abandoned workspaces.
Shortly after 4pm 40 to 50 journalists left after handing in their badges.
"It's sad, but I'm also really proud of the press corps, that we stuck together," said Nancy Youssef, a reporter for The Atlantic who has had a desk at the Pentagon since 2007.
Press 'disruptive' to world peace
It is unclear what practical impact the new rules will have and news organisations have vowed to continue covering the military regardless.
Images of reporters demonstrating against barriers to their work are unlikely to move supporters of President Donald Trump.
Mr Trump has been involved in court actions against the New York Times, CBS News, ABC News, the Wall Street Journal and the Associated Press in the past year.
Speaking to reporters at the White House on Tuesday, the US president backed Mr Hegseth's rules.
"I think he finds the press to be very disruptive in terms of world peace," Mr Trump said.
"The press is very dishonest."
Hegseth blocked flow of information
Even before issuing the policy, Mr Hegseth, a former Fox News Channel host, has systematically choked off the flow of information.
He has held only two formal press briefings, banned reporters from accessing many parts of the sprawling Pentagon without an escort and launched investigations into leaks to the media.
Mr Hegseth said the requirement that journalists sign a document meant they acknowledged the new rules, but not that they necessarily agreed to them.
Journalists saw that as a distinction without a difference.
"What they're really doing, they want to spoonfeed information to the journalist and that would be their story," retired US Army general and Fox News analyst Jack Keane said on Mr Hegseth's former network.
"That's not journalism."
The Pentagon Press Association, which has 101 members representing 56 news outlets, has spoken out against the rules.
Organisations from across the media spectrum told their reporters to leave instead of signing the new rules.
ABC/AP
