Japanese internment camps from WW2 easily meet that definition.
I think immigration detention centers easily meet that definition too: they hold large numbers of individuals whose only crime is being “the other”.
"Being illegally present in the U.S. has always been a civil, not criminal, violation of the INA[Immigration and Naturalization Act]"
"Criminal violations of the INA, on the other hand, include felonies and misdemeanors and are prosecuted in federal district courts. These types of violations include the bringing in and harboring of certain undocumented aliens, aliens (INA §275), ..."
As can already be deduced from the above, illegal entry is a misdemeanor. Only the bringing in, harbouring, and certain specific aggravation conditions raise it to a felony.
The _concentration_ part is missing. Without condoning anything of whatever wrongdoing ICE is doing all the people detained decided personally to embark in a trip that involved that risk.
Concentration camps need to have a concept of raking up a chunk of the population and removing them from society, as in concentrating a part of the population. Prisons are more of a concentration camp than ICE detention centers.
Also misdemeanors are still considered serious crimes. I don't get where this hand-waving "because it's just a misdemeanor" comes from.