The US started out as a hodgepodge of kludgy compromises, with a uselessly weak confederate structure, but after a dozen years of experience, the system was in many ways failing. And so, still wanting to make the nation hold together and function, with the memory of winning independence fresh, the states decided to start over, and gathered a group of delegates to write a new constitution establishing a more coherent and unified federal government.

At this point I think internationalism and republicanism in Europe is sufficiently weakened that such an outcome is impossible in the near future.