I have never seen a breach quite like this. It is not uncommon for national-security officials to communicate on Signal. But the app is used primarily for meeting planning and other logistical matters—not for detailed and highly confidential discussions of a pending military action. And, of course, I’ve never heard of an instance in which a journalist has been invited to such a discussion.
I wrote a piece to contextualize this story in the context of the types of discussions that take place on privacy-focused tech forums:
TL;DR: This was a human error. Signal (nor any other E2EE app for that matter) could not have prevented this, because E2EE smartphone apps are simply not the right tool for the job of enabling government officials to securely conduct military operations.