The sentence is simple because it doesn't have any conjunctions or complex-sentence-making words.
Description; because if you didn't know what eluded meant you could understand by reading the description of what is/ has happened.
By giving military commanders the authority to determine when, where, and how long a person needs to be detained, President Roosevelt is assuming that <span>all the commanders will move forward fairly and with sound judgment. This is what he says:
</span><span><em>"...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..."</em></span>
