Whereas the financial press may date the end of the US as we know it to the 1980s, when terminal bankruptcy was trumpeted as a sign of bogus immortality, the layperson on the ground is more likely to date the desovereignization process to the breakdown of borders in the Middle East.
As of 2014, no one really knows where the borders are among the former nations of Syria, Jordan, Israel, Palestine, Iraq and so on. Political maps survive, showing snap shots of various people's belief systems, however the credibility of such was draining away from the 1940s onward.
Ironically, a chief catalyst in the breakdown of the nation-state system was the Pentagon's "strategy" of bombing whomsoever it pleased in the No Man's Land around Afghanistan-Pakistan. The cruise missile and later drone attacks were corrosive of the whole sovereignty concept and softened up that framework for eventual garbage collection.
Of course Romans did not really know Rome as an Empire had "fallen" on any given day. Those narratives get invented later and only hold water through repetition and agreement, as stories are much more than necklaces of facts strung together. The infrastructure of passports, customs, import / export taxes and so on remains, especially in areas where borders are enforced.
The end of the nation-state system happened first on paper in the back office, where those in the know were holding high cards or at least knew what a "high card" was. Then "border rot" propagated by Pentagon policies, helped the world's "average joes" come to terms with the fragility of the entire nation-state framework. Life would go on amidst the rubble of once proudly recognizable jurisdictions.