Every time the Military started to do anything, an ACLU attorney would run get a restraining order.
Normally, these type of trials take place after the war is finished. Witness the Nurenburg Trials.
There were German spies and prisoners held for many years until WWII was finished.
There were German spies and sabateurs captured early after the US entry into WWII in 1942. They were put on trial by military tribunals within weeks of their capture. They were convicted and executed within a couple of months. It was FDR's idea to hold military tribunals rather than civilian trials.