Perhaps the war they waged on Chassidus did harm to their future, rather than "saving Chassidus from itself..."
This is not to say that chassidim did not experience some of that assimilation, but it would be interesting to know where the rates of assimilation were higher.
claims regarding this are made about the early rounds of haskala (reshima posted by
@ExGingi ) but even there not for that reason.
You may be thinking of the burning of the ספרי הרמב"ם and the שריפת התלמוד
(We can all make our assumptions of which countries were most strongly affected, and why,) but I don't know if it's fair -or productive- to bring that up without anything to back it up.
The one thing you can safely say, as the Rebbe mentioned multiple times, that the derech of Chassidus Chabad has proven itself especially resilient in the face of extended communist oppression.
(requiring actual מסירת נפש )