I don't suspect the United States (singularity) was ever a sovereign nation in the organic sense--it seems to most always have been the creature of sovereign states. If we go back to the Articles of Association, the United States was a creature of colonies. Consider the case for hen's teeth. Many defending it and seeking to protect but it doesn't exist in the first place. The confederacy named "The United States of America" itself was sovereign only relative to the territory afforded her by the sovereignties which comprised her. The United States's sovereignty was never in respect of its creators but only with respect to that over which it was is sovereign. While the United States might have relative sovereignty with respect to its territories, possessions or districts--it has never had more and can lose no more than it has any more than your neighbor can lose your shoes that he doesn't have. The 50 States of the United States are subdivisions of the United States like counties are to a state. The district courts are akin to state circuit courts.
The sovereignty the United States has with respect to its territories/districts/possessions is the only sovereignty that it ever had. The District of Columbia is a territory for all practical purposes which is why Congress has plenary power over it.
During Shay's Rebellion, Geo. Washington sent troops into the district of Pennsylvania. He did not send them into de jure and organic Pennsylvania land.