Fredrich Engels, 1872: On authority

Have these gentlemen ever seen a revolution? A revolution is certainly the most authoritarian thing there is. It is the act by which one part of the population imposes its will on the other part by means of rifles, bayonets and cannons — by the most authoritarian means possible; and the victors, if they do not want to have fought in vain, must maintain this rule by means of the terror which their arms inspire in the reactionaries. Would the Paris Commune have lasted a single day if the communards had not used the authority of the armed people against the bourgeoisie? Should we not, on the contrary, reproach them for not having used it enough?

Therefore, we must conclude one of two things: either the anti-authoritarians don’t know what they’re talking about, in which case they are only sowing confusion; or they do know, in which case they are betraying the proletarian movement. In either case, they serve reaction.

At what point does a leftist system become authoritarian? Where is the line? Is it just a vibe check, or is there a definitive metric we can check?