Reagan would have given ONE warning, after the Lusitania (1915) and I bet we would have been in the war the next year, because the Germans had no intention of heeding any warnings.
Unlike you, Reagan was a great respecter of the Constitution.
The president has no authority to declare war.
I do agree that he would have cajoled congress, who does have the constitutional authority, into declaring war. And probably faster than Wilson, too.