It has recently emerged that, in 1950, at the outbreak of the Korean War, Hoover advanced a plan to suspend the writ of habeas corpus and consign 12,000 people, the vast majority of whom were putatively disloyal American citzens on whom the FBI had been keeping very close watch, to indefinite terms of imprisonment:
The man was a visionary. Of course, he did have the template of the Palmer Raids to work from.
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.
“In order to make effective these apprehensions, the proclamation suspends the Writ of Habeas Corpus,” it said.
The prisoners eventually would have had a right to a hearing under the Hoover plan. The hearing board would have been a panel made up of one judge and two citizens. But the hearings “will not be bound by the rules of evidence,” his letter noted.
Sadly for our existentially threatened country, Harry Truman was a complete wuss, and he didn't go for this idea, and so, you know, the Communists took over and stuff.
Hoover Planned Mass Jailings in 1950 (New York Times, 23 December 2007)