In World War II, the Japanese kept a secret camp called Ofuna:
This wasn’t a POW camp. It was a secret interrogation center called Ofuna, where “high-value” captured men were housed in solitary confinement, starved, tormented, and tortured to divulge military secrets. Because Ofuna was kept secret from the outside world, the Japanese operated with an absolutely free hand. The men in Ofuna, said the Japanese, weren’t POWs; they were “unarmed combatants” at war against Japan and, as such, didn’t have the rights that international law accorded POWs. In fact, they had no rights at all. If captives “confessed their crimes against Japan,” they’d be treated “as well as regulations permit.” Over the course of the war, some one thousand Allied captives would be hauled into Ofuna, and many would be held there for years.
— Unbroken – by Laura Hillenbrand (Kindle Locations 3055-3059)
After World War II, at the trial of the Nazis:
Justice Jackson: Protective custody meant that you were taking people into custody who had not committed any crimes but who, you thought, might possibly commit a crime?
Hermann Goering: Yes. People were arrested and taking into protective custody who had not yet committed any crime, but who could be expected to do so if they remained free … the original reason for creating the concentration camps was to keep there such people whom we rightfully considered enemies of the state.
— The Trial of Hermann Goering, Nuremberg, 1946
From Wikipedia on Guantanamo:
The Guantanamo Bay detention camp, also referred to as Guantánamo or Gitmo, is a United States military prison located within Guantanamo Bay Naval Base, which fronts on Guantánamo Bay in Cuba. At the time of its establishment in January 2002, Secretary of Defense Donald H. Rumsfeld said the prison camp was established to detain extraordinarily dangerous prisoners, to interrogate prisoners in an optimal setting, and to prosecute prisoners for war crimes. Detainees captured in the War on Terror, most of them from Afghanistan and much smaller numbers later from Iraq, the Horn of Africa and Southeast Asia were transported to the prison.
After Bush political appointees at the U.S. Office of Legal Counsel, Department of Justice advised the Bush administration that the Guantanamo Bay detention camp could be considered outside U.S. legal jurisdiction, military guards took the first twenty detainees to Guantanamo on 11 January 2002. The Bush administration asserted that detainees were not entitled to any of the protections of the Geneva Conventions.