alternatehistory.com

To identify culprits and assign responsibilities, you need witnesses, investigators, reporters, and authorities; when an incident is politicized enough, witnesses, investigators, reporters, and authorities ("WIRA") are not immune to partiality and ulterior motives. When an entity is powerful enough, it can have enough "WIRA" confirm and reiterate its own narrative (and accuse its enemy of perjury, forgery, and whatnot)- that's how Russia has convinced millions worldwide that there is no Russian invasion of Donbass. Had the Russians perfected their propaganda decades earlier, they could have denied culpability in the Winter War (no Soviet invasion), Cuban Missile Crisis (no Soviet deployment), the Prague Spring (no Soviet invasion), and other incidents.

In general, any involvement short of deployment in need of general mobilization is deniable.

I'm not sure about Operation Barbarossa - was it necessarily undeniable on the part of the Germans?
Top