Marathe & Steeply: the threat is homegrown — someone killed your nation — "Someone or some people among your…"
Someone or some people among your own history sometime killed your U.S.A. nation already, Hugh. Someone who had authority, or should have had authority and did not exercise authority. I do not know. But someone sometime let you forget how to choose, and what.
Context
The pickup's core, and the theme's deepest cut: the weapon is not Québécois but American ("a U.S.A. production... The appetite... this also is U.S.A."), so the real threat was internal long before the samizdat — "someone sometime killed your U.S.A. nation already." The diagnosis is political, not moral: a failure of "authority" that "let you forget how to choose," a nation hollowed from within by its own abdication. Québec did not kill America; America's own history did, and the Entertainment only reveals the corpse. (The spectation/appetite and choosing-your-master philosophy are entertainment-self's; landed here is the national-diagnosis substance the plan routes to us.)