Question

By giving military commanders the authority to determine when, where, and how long a person needs to be detained, president roosevelt is assuming

Answer 1

By giving military commanders the authority to determine when, where, and how long a person needs to be detained, President Roosevelt is assuming that all the commanders will move forward fairly and with sound judgment. This is what he says:
"...whenever he or any designated Commander deems such action necessary or desirable, to prescribe military areas in such places and of such extent as he or the appropriate Military Commander may determine..."