Article I, Section 8 of the United States Consitution.
"It provides for Congress to declare war, but I don't see where it requires the Executive to get a declaration."
What would be the point of this provision if it didn't require the Executive to get a declaration?
How should I know? Diplomatic reasons? International protocol? Fact is, from the standpoint of strict construction, I can't find any requirement in the Constitution along the lines of "Congress shall authorize no troops without a declaration of war." That would settle the matter, 'cept it ain't there.