"Hoover sent his plan to the White House on July 7, 1950, 12 days after the Korean War began. It envisioned putting suspect Americans in military prisons.
Hoover wanted President Harry S. Truman to proclaim the mass arrests necessary to “protect the country against treason, espionage and sabotage.” The F.B.I would “apprehend all individuals potentially dangerous” to national security, Hoover’s proposal said. The arrests would be carried out under “a master warrant attached to a list of names” provided by the bureau.
The names were part of an index that Hoover had been compiling for years. “The index now contains approximately twelve thousand individuals, of which approximately ninety-seven per cent are citizens of the United States,” he wrote."
Back when I was writing my second book Against Gravity, I thought it would take terrorists nuking Los Angeles to persuade the US government of the late 21st Century to carry out mass arrests of its own citizens. Apparently it doesn't take that much at all, if the details of a newly declassified account (found via Boing Boing) are anything to go by: