This has been very useful to me since I encountered it on the old sub. Within the last years, war has broken out. War seeds the clouds of formerly clear skies. The droplets of propaganda are now a torrent.
As such, I don't think it completely wise to follow this rule unaltered when states clash. The power, organization, and incentives come together to cloudy any former clarity. Lies, with a multitude of motivations, float freely.
What addendums would you add to this adage in an age of active conflict?
I think the better version of this phrase is "when someone *shows * you who they are, believe them."
We're all the hero of our own story and someone can tell you all day long that they're a nice person and that they're always being taken advantage of...but if you see them being an inconsiderate prick and watch them engineer a situation that leads to their own immiseration you might pause and consider what the other perspective of all their stories actually looks like.