From Merriam Webster: "TREATY: a contract in writing between two or more political authorities (as states or sovereigns) formally signed by representatives duly authorized and usually ratified by the lawmaking authority of the state"
From The European Treaty-Constitution and Constitutional Identity: "...a constitution establishes, modifies or reestablishes a common unit with an identity of its own; a treaty in contrast, carves out a discreet common project among sovereign parties with each retaining its own distinct identity."
From the 1856 Bouvier Law Dictionary: "TREATY, international law. A treaty is a compact made between two or more independent nations with a view to the public welfare treaties are for a perpetuity, or for a considerable time. Those matters which are accomplished by a single act, and are at once perfected in their execution, are called agreements, conventions and pactions.
Oh, I forgot. You subscribe to the Lincolnian nonsense about the Union preceding the states.
I believe the Articles of Confederation were more "treaty-like" in their structure; whereas the Constitution of 1787 was more "contract-like."