Stalin’s purges were a standing institution, not response to circumstance.

Evidence

However little we know of the details of this story, we know more than enough to support my original conviction that such “wrecking operations” as the Great Purge were not isolated episodes, not excesses of the regime provoked by extraordinary circumstances, but that they were an institution of terror and to be expected at regular intervals—unless, of course, the nature of the regime itself was changed. — Hannah Arendt, The Origins of Totalitarianism, 54