National Sovereignty is necessary in order for the people of the United States, both individually and collectively, to maintain their liberty. The purpose of government, and the ONLY purpose of government, is to secure the rights of life, liberty, and pursuit of happiness of the people. The Constitution of the United States was created for that purpose only, and grants government officials only specifically limited powers. One of those powers is to make treaties with other nations, but those treaties must be made "under the authority of the United States." Just like statutory laws, treaties that infringe on the unalienable rights of the people, or that are inconsistent with other Constitutional restrictions, are null and void.
While international good will and cooperation is desirable and a benefit to all people, any agreements made must be in accordance with the preservation of the people's rights. Our policy should be as Jefferson advised, friends and trading partners with all nations, entangling alliances with none.