As promised, starting a new thread tonight, to tell you how, instead of falling into well-deserved oblivion, concentration camps persisted into the 20th century. A simple answer is that once established, a military tactic or tool rarely vanishes.
British law on enemy aliens had for centuries been expansive—tremendous latitude was given with regard to treatment of individual aliens in wartime. But ahead of WWI, recent anti-immigrant statutes had particularly ramped up fear and resentment of foreigners.
When war broke out, as was typical, several aliens suspected of being spies and saboteurs were rounded up by belligerent nations. But this time, a cry arose to lock up up enemy aliens—citizens or subjects of opposing countries—as a group. All of them.
The British home secretary resisted these calls for months (see text image). But after the sinking of the Lusitania by a German U-boat and the death of innocent civilians, it became impossible to ignore demands to do *something,* even something pointless and unrelated. 
The British began rounding up tens of thousands of enemy alien civilians and putting them into concentration camps. The policy was expanded across the British Empire. Germany did the same. As other belligerents entered the war, they, too, set up camps.
By the end of the First World War, nearly a million people around the globe were held in what we would today call internment camps (which were generally known as concentration camps at the time). The Red Cross sent observers to monitor conditions in them.
Bureaucracies of detention emerged. Postal systems were set up. What had been a collection of ragged, brutal concentration camp systems in colonial outposts at the turn of the century was transformed, "civilized," and brought into the heart of Europe.
Civilians learned to register themselves, to appear at an appointed location when asked, to get a prisoner number, & to wait out their time as a member of a pariah class until they were released. In turn, the public came to believe that concentration camps were humane & necessary
After WWI, camps were an acceptable institution for civilians seen as a threat. Indefinite detention was used by the British against the Irish, during the Russian Civil War, & worldwide to deal with social unrest or members of society deemed undesirable.
The camps of WWI normalized the idea of mass civilian detention without trial around the world. It's impossible to get to Japanese American internment, to the Soviet Gulag & especially to the Holocaust without them. Tomorrow I'll talk about how camps evolved before & during WWII.
