We had a varsity team, but we needed a plan.
.American and European Unions, Compared(1)
The (1648) Treaty of Westphalia created the modern nation-state by respecting sovereignty within agreed boundaries. Soon, everyone had a sovereign King. Today, Europeans live in republics, but wish to unite for economic benefits. Like others however, they found that hard to do, and so began with monetary union, alone. Unlucky timing: world monetary crisis suddenly struck.
The Annapolis Convention had the original purpose of negotiating the joint ownership issues of the Potomac River between Virginia and Maryland. It seemed quite a natural thing to extend the discussion to other rivers which needed interstate cooperation, particularly the Susquehanna (Maryland and Pennsylvania) and the Delaware (Pennsylvania, New Jersey, Delaware and New York). That was at least six of the thirteen colonies, where Maryland and Virginia would find uniformity of rules to be advisable. It did not take too much imagination to suppose that the remaining states would wish to be involved, as well. It seems that much of the push for enlarging the local river discussion into a national discussion of broader issues, came from Alexander Hamilton.