Each state of the USA is large enough to be its own country. It doesn't work equally well for all states, but there is a high degree of freedom of movement, and

Each state of the USA is large enough to be its own country. It doesn't work equally well for all states, but there is a high degree of freedom of movement, and free trade between states. If every state were indeed its own country, it seems unlikely there would only have been one war between them since 1861.