On 17 September 1787 the Constitution of the United States was signed by the delegates of the convention meeting in Pennsylvania.  It would be many months of long debate before the Constitution was ratified.

American military officers are perhaps unique in their sworn allegiance not to their commanders, and not to the President, but to the US Constitution:

I…do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign or domestic, that I will bear true faith and allegiance to the same…

Though amended through time, the US Constitution represents both the founding and continuation of a free nation without peer in the world.