The Byzantine Generals Problem Leslie Lamport, Robert Shostak, and Marshall Pease (1982) ACM Transactions on Programming Languages and Systems, Vol. 4, No. 3, July 1982, Pages 382-401 https://lamport.azurewebsites.net/pubs/byz.pdf
From the abstract: ... It is shown that, using only oral messages, this problem is solvable if and only if more than two-thirds of the generals are loyal; so a single traitor can confound two loyal generals. With unforgeable written messages, the problem is solvable for any number of generals and possible traitors. ...
Clearly this hack (and other prior crypto hacks) demonstrates that the 'Unforgeability' condition is practically impossible due to security implementation weaknesses. One can never rule that out entirely. That leaves no less than 2/3rds of the network as the bare minimum for reasonable consensus.
Lamport's paper is from 40 years ago and blockchains/systems that ignore these theoretical foundations are doomed to repeat the same flaws again and again!