The order to authorize internment camps was the result of two forces:
* Intense and direct recommendations from FDR's entire war staff and intelligence apparatus, which believed that there was an unacceptably high probability that Japanese and German populations would engage in sabotage during the war
* ENORMOUS public pressure, particularly in California, to eject Japanese citizens
The Japanese internment debacle is a blemish and a tragedy, no doubt, but it's not an illustration of an executive power grab. The whole country shares the blame for it.
The lesson I take from it is the same as the lesson I take from drone strikes blowing up wedding parties: Congress should be extraordinarily careful and guarded in its authorizations for using military force of any sort.