I beg to disagree. The Union could not have been created without the Constitution. It was the Ratification of the Constitution that created the Union of States that now exist. The agreement to abide by that Constitution.
Yes, I am aware that there was a Union prior to the Creation of this Constitution under the Articles of Confederation. And it was in an effort to maintain that Union that was the reason for the writting the NEW Constitution. That Union under the Articles of Confederation was in the process of disbanding if a new agreement could not have been reached and none of the 13 states at that time were bounded to join together under that new Constitution. Therefore if that Constitution had not been agreeable in its terms and limitation placed upon the central government under that Constitution to those states, there would not have been a Union at all.
The writting and adoption of our Constitution it was a last desperate attempt to preserve a form of Union between the several states with were highly inclined to govern themselves as independent nation states.