They used knowledge of the social graph to proceed clique by clique, triangulating such that their point of view appeared to come from multiple independent outside sources. In 1917.
(see my comment below about "identity" being irrelevant to bad actors)
as to identity: http://www.gutenberg.org/files/48612/48612-h/48612-h.htm notes
"First point to note is the character of the source. There are several choices on this: the true source (who really got it out?) and the ostensible source (whose name is signed to it?); also, the first-use source (who used it the first time?) and the second-use source (who claims merely to be using it as a quotation?). Take the statement: "Harry said to me, he said, 'I never told anybody that Al's wife was a retired strip-teaser.' Mind you, I don't pretend to believe Harry, but that's what he said, all right." What are the possible true sources for the statement of fact or libel concerning Al's unnamed wife? What are the alternatives on ostensible sources? First use? Second use? The common sense needed to analyze this statement is of the same order as the process involved in analyzing the statement: "Reliable sources in Paris state that the visit of the American labor delegation has produced sensational repercussions in Moscow, and that Moscow, upon the basis of the American attitude, is determined to press for unification of the entire German labor movement.""
and more directly to your point (that overt identities may not even matter), I would say Zweig's Schachnovelle is an extended meditation on parties who continue politics by openly unlawful yet unopposed force.
It's the wrong metaphor. The Maginot line was supposedly impenetrable for keeping clearly labelled troops on the other side. The modern situation is something much more porous, much more like Cold War (counter)intelligence. Or this discussion of "interpretative superiority": http://www.harrowell.org.uk/blog/2018/02/18/why-corbyn-is-th... (although I disagree with the conclusion). A system that can detect ideological incursions, vector an intercept response, and shoot down memes that get over the line. Analogy between forum moderation and those Phalanx CIWS systems for shooting down missiles.
The West doesn't have a central ideological defence system though, or at least not since the Cold War. Instead it's more like an immune system. Individual agents wander around the body politic attacking ideas they find unfamiliar. Responses to ideological challenge happen not through central coordination but by the actions of individual and factions.
The effectiveness of this system has become an autoimmune disease that can be exploited by pathogenic actors. The US police are a good example of this: their ideological defence is so strong it can cover even crimes committed in broad daylight on video.