Trump says he knows nothing of Atlantic journalist swept up in war plan text chain
thehill.com

... Trump was asked about the report during an event with Louisiana officials at the White House shortly after it surfaced. The president maintained he was no fan of the publication and said he had no knowledge of the editor being accidentally included in the chain.

"I don't know anything about it. I'm not a big fan of The Atlantic; to me it's a magazine that is going out of business," Trump said. "I know nothing about it. You're saying that they had what?"

A reporter responded that Trump officials were using Signal to coordinate on sensitive material and when Trump questioned "having to do with what?" the reporter said, "the Houthis." Trump replied, "You mean the attack on the Houthis?"

"Well, it couldn't have been very effective, because the attack was very effective, I can tell you that. I don't know anything about it. You're telling me about it for the first time," Trump added. ...

SIPRNet
en.wikipedia.org

... The Secret Internet Protocol Router Network (SIPRNet) is "a system of interconnected computer networks used by the U.S. Department of Defense and the U.S. Department of State to transmit classified information (up to and including information classified SECRET) by packet switching over the 'completely secure' environment".[1] It also provides services such as hypertext document access and electronic mail. As such, SIPRNet is the DoD's classified version of the civilian Internet.

SIPRNet is the secret component of the Defense Information Systems Network.[2] Other components handle communications with other security needs, such as the NIPRNet, which is used for nonsecure communications, and the Joint Worldwide Intelligence Communications System (JWICS), which is used for Top Secret communications. ...

Also, conversations on SIPRNet are archived, according to law about government conversations. From what I understand, this conversation on Signal was tagged to self-delete in a couple of weeks.

The latter was a concern in the "... but her emails..." case.

Five Eyes
en.wikipedia.org

... The Five Eyes (FVEY) is an Anglosphere intelligence alliance comprising Australia, Canada, New Zealand, the United Kingdom, and the United States.[1] These countries are party to the multilateral UKUSA Agreement, a treaty for joint cooperation in signals intelligence.[2][3][4] Informally, "Five Eyes" can refer to the group of intelligence agencies of these countries. The term "Five Eyes" originated as shorthand for a "AUS/CAN/NZ/UK/US Eyes Only" (AUSCANNZUKUS) releasability caveat.[5]

The origins of the FVEY can be traced to informal, secret meetings during World War II between British and American code-breakers that took place before the US formally entered the war.[6] The alliance was formalized in the post-war era by the UKUSA Agreement in 1946. As the Cold War deepened, the intelligence sharing arrangement was formalised under the ECHELON surveillance system in the 1960s.[7] This system was developed by the FVEY to monitor the communications of the Soviet Union and Eastern Bloc; it is now used to monitor communications worldwide.[8][9] The FVEY expanded its surveillance capabilities during the course of the "war on terror", with much emphasis placed on monitoring the Internet. The alliance has grown into a robust global surveillance mechanism, adapting to new domains such as international terrorism, cyberattacks, and contemporary regional conflicts. ...

OK, after reading that, and taking a step back ...

After what seemed to have been leaked in this Signal (I'll be kind) faux pas, what will those other Nations of Five Eyes now think about sharing intelligence with the US?

Top Trump officials text classified Yemen airstrike plans to journo in Signal SNAFU
www.theregister.com

... Military analyst and retired Australian Army Major General Mick Ryan has described the use of the secure chat app in this matter as "appalling."

"By using Signal for such a sensitive issue, the participants demonstrate a cavalier attitude to operational security," he wrote Tuesday, Australian time. "For a Secretary of Defense who allegedly values a war-fighting ethos, this shortfall in security is appalling. In normal times, this would see people sacked. I don't expect that in this case though because these are not normal times."

"Why aren't they using more secure communications that are assured by the NSA or another government communications agency?" he added. The Register understands chat tools used by those agencies include features that prevent, say, journalists from participating in group chats.

Another question worth asking is why the group's members used Signal, and why they set at least some of the messages to auto-delete, which may be a violation of federal records-keeping laws.

Using Signal is also extraordinary given that a constant refrain of President Trump's 2015 presidential election campaign was the need to "lock up" Hillary Clinton for using a private server for her emails.

As Ryan pointed out: "By communicating on devices which were transmitting information about a future operation in real time to a journalist, the conversation may also have been compromised by other foreign interested parties. As a result, might the entire operation to strike the Houthis have been compromised, and the lives of military personnel put at risk?" ...

www.cbsnews.com

... The National Security Council stated that the messages seem to be "authentic" in a statement to CBS News after the story was published.

"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," a National Security Council spokesman said.

"The thread is a demonstration of the deep and thoughtful policy coordination between senior officials. The ongoing success of the Houthi operation demonstrates that there were no threats to our service members or our national security." ...