Counterfactual debate: If Congress had not declared war in WW II

David Nieporent has made the amazing claim that, if Congress had voted not to declare war against Japan in World War II, the U.S. still would have been at war with Japan. This boggles my mind. I don't see what the purpose of the Founders writing into the Constitution that only Congress had the power to declare war, if even if Congress blatantly refused to declare war, we would still be at war.

 

This is an invitation to Mr. Nieporent to comment on why he thinks that the U.S. can be at war even if Congress explicitly refuses to declare war.