The Geneva conventions weren't adopted till 1949, so they didn't have protections under those international laws.
Do you honestly think those scumbags in Guantanamo don't know why they are being held? They are terrorist, even under Geneva conventions now if your not in a military uniform you don't have Geneva rights.
We killed 100's of German spies right there on the spot during WW2, why? Because the enemy you don't kill today might be the enemy that kills you tomorrow. That was one of the major points in Saving Privet Ryan, when the German, the one soldier let go came back and killed him and his friend.