It's been blasted as one of the most "stunning and dangerous" breaches of military intelligence ever seen.
Senior Trump officials set up a group chat to discuss top secret Yemen war plans.
But they accidentally looped in a journalist.
The National Security Council says it's investigating, but the breach has sparked widespread outrage over how classified information is being handled.
So what have we learned?
What was leaked to the journalist?
The chat, named "Houthi PC Small Group", was set up on the Signal messaging app on March 13.
It appeared to include cabinet secretaries and top aides, including Vice-President JD Vance, Defence Secretary Pete Hegseth and national security adviser Mike Waltz.
But what the group didn't know, was that they'd unknowingly looped in Jeffrey Goldberg, editor-in-chief of magazine The Atlantic.
A day after he was added, the group started sharing information about plans for a military operation against the Iran-aligned Houthi rebel group.
The chat included details of air-strike targets and timing.
The name of an active CIA intelligence officer — which is usually kept confidential — was also shared.
In the discussion, the group debated whether to launch the strike in response to Houthi attacks of Red Sea shipping.
They exchanged critical comments about Europe, arguing their allies across the ocean benefited economically from the US navy’s protection of global shipping routes.
The chat also included comments from Vice President JD Vance, who appeared sceptical about the urgency of attacking Yemen.
How did the journalist respond?
Mr Goldberg has detailed his stunning experience in an article titled The Trump Administration Accidentally Texted Me Its War Plans
He said he initially doubted the group text was real.
"I could not believe that the national-security leadership of the United States would communicate on Signal about imminent war plans," Goldberg wrote.
"I also could not believe that the national security adviser to the president would be so reckless as to include the editor in chief of The Atlantic."
But just two hours after Mr Goldberg received the texts, on March 15, the US attack on Houthi targets began.
The ABC has confirmed the authenticity of the group chat with the White House's National Security Council.
"At this time, the message thread that was reported appears to be authentic, and we are reviewing how an inadvertent number was added to the chain," spokesman Brian Hughes said in a statement.
How has the Trump administration responded?
Defence Secretary Pete Hegseth claims it's just more "garbage" reporting from Mr Goldberg.
"You're talking about a deceitful and highly discredited so-called journalist who's made a profession of peddling hoaxes time and time again," he said to journalists on Monday.
"Nobody was texting war plans. And that's all I have to say about that."
Shortly after the story was published, President Donald Trump told reporters: "I don't know anything about it."
"I'm not a big fan of The Atlantic," he said.
"To me, it's a magazine that's going out of business."
How have others responded?
The story has triggered widespread ire from Democratic lawmakers, with calls for a full investigation.
“This is one of the most stunning breaches of military intelligence I have read about in a very, very long time,” Senate Democratic leader Chuck Schumer said in a floor speech on Monday.
Senator Jack Reed, the top Democrat on the Senate Armed Services Committee, has also spoken out.
"If true, this story represents one of the most egregious failures of operational security and common sense I have ever seen," he said.
"The carelessness shown by President Trump’s cabinet is stunning and dangerous."
Senate Majority Leader John Thune, a South Dakota Republican, said he wants to learn more about what happened.
"Obviously, we got to to run it to the ground, figure out what went on there," he said.
Responding to The Atlantic article on social media platform X, former Democratic presidential candidate Hillary Clinton wrote, "You have got to be kidding me."
In the lead-up to the 2016 US presidential election, Ms Clinton was investigated by the justice department for using a private email server to communicate with aides about classified information.
The FBI ultimately recommended against charges and none were brought.
Do officials commonly use Signal?
During the Biden administration, some officials were permitted to download Signal on their White House-issued phones.
But a former national security official who served in the Democratic administration told the Associated Press they were instructed to use the app "sparingly".
They said Signal was most commonly used to notify people who were out of office that they should check their "high side" inbox for classified messages.
The official said use of the app surged during the last year of the Biden administration, as federal law enforcement officials warned hackers from China and Iran were targeting the White House.
But they were unaware of any top Biden administration officials using Signal to discuss sensitive plans.
Why is this incident concerning?
David Smith, an associate professor of American Politics and Foreign Policy at the United States Studies Centre, said highly classified conversations like these should never take place on apps like Signal.
"The reason why these security procedures are in place is to protect Americans," he told the ABC.
"It is disastrous that this was allowed to happen, even if it didn't have any consequences."
He said this conversation was illegal on two counts.
"First, they were not using secure government systems," he said.
"Second, they were using software which automatically deletes messages that are required by law to be kept.
"Then, of course, the truly catastrophic thing is that a complete outsider ... was inadvertently added to the chat."
Mr Smith said others were likely using Signal to discuss classified matters.
"One reason might be that they they find it a lot easier and quicker just to communicate with each other this way," he said.
"It also keeps that communication away from anybody else in in the government."
What's happens next?
Probably not much, according to Mr Smith.
The National Security Council will look into this incident, and likely investigate whether Signal and other apps are being used by others to discuss classified matters.
As for the officials involved in this chat, Mr Smith said it's unlikely there'll be any consequences.
"It would be up to the Department of Justice to investigate and prosecute," he said.
"I see no chance that [they] would launch an investigation or a prosecution that would embarrass the Trump administration in this way."
Who owns Signal?
Signal is an encrypted messaging app owned by the Signal Foundation, an American non-profit organisation founded in 2018.
On its website, it says its mission is to "protect free expression" and enable "secure global communication through open source privacy technology".
