If you want to get technical, once the President assumes the Role of Commander in Chief of the United States Military he has control over all military action while congress; per the original intent of the constitution, has control over starting war and ending it. If you really had to choose one or the other the Commander in Chief has in most occurrences decided how war ends. I really hope this helped, if not my apologies and if you need any more elaboration please let me know down in the comments and I will do my best.
